Hi all and thanks for reading.
Hoping to get advice before I spend $$$ on the wrong parts.

3rd owner of '67 Conv which has a past I don't fully understand, but it has been restored.
Someone along the way pulled the smaller original motor and replaced it with a '68 400. (400 was rebuilt and runs flawlessly).
Original TH 350 was left in place, diff gears switched to 3.08 (I am told), and she drives like a 3.08 which fits my needs.
So .... she really needs a manual 4-speed Muncie m20 wide ratio if I'm to do the car right as her owner.

I have access to a beautifully rebuilt early '70 m20 wide ratio 10/27 spline, 661 case.
The '68-69 m20's I can find are shoddy or "unknown" rebuilds, but do have the 660 case that was age matching.
Will the '70 m20 bolt onto my '68 400 in my '67 frame ?
I know my TH350 cross member is a match for the m20.
Is the case difference between 660 and 661 is just going from male to female shift connectors ???
Planning on new flywheel, bell housing, clutch assembly, etc.

Was also told to get the Hust Comp shifter for '67-68 (to fit the "67 frame) .... and a shifter linkage kit for '69-70 Muncie to fit the '70 m20 with female shift posts. I am sure it will all fit together perfectly the first time ......

I know there are sure to be glitches to address with the swap and I am mixing years (it's already been done with the '68 400) but everything I read says the '70 Muncie m20 will work, and the wide ratio with a steep first gear is the correct match with my 3.08 diff.

Am I barking up the right tree with the "70 m20 as far as a compatible match to my '68 400 ?
I am only interested in an age appropriate 4-speed Muncie (not a newer transmission) to keep the era vibe correct for her.

Thanks for any advice or pitfalls that you have suffered through.

Sounds like a nice car. Have you decoded the numbers? A T-350 was not used until 1969. In '67 it was a 2 speed automatic used with an OHC 6 cylinder or 326, or a T-400 if it had a 400 engine. [A buddy's car was a 1967 400 w/T-400 and 3:08 rear gears. A great combination] Later low compression 400's used the T-350. Any Saginaw, Muncie, or Borg Warner 3 or 4 speed manual trans can be used with the 2 speed automatic/ T-350 cross member, the T-400 is different. They will all bolt up to the BOP manual bell housing. Get a new forged steel flywheel. Most used factory cast iron units will not pass a magnaflux test. Steel is safer and won't explode. With 3:08 gears [great for highway mileage] you need a manual trans with a decent first gear. I try to shoot for a 10 to 1 overall ratio multiplying the first gear with the rear gear. That is best for easy take off especially in traffic. Most Muncie trans have either a 2:20, 2:52, or 2:56 first gears. They need 3:73, or 4:11 rears to work well. The more common wide ratio 2:52 multiplied by the 3:08 rear gets you 7.76 overall ratio. You will be riding the clutch just to get going even with a 400. I had a 3:08 rear in my 1977 TA, 400 engine, but with a Super T-10 trans with a 3:42 first gear. Overall ratio was 10.5 to 1 and I could climb a tree with that car. At 60mph it was 2000 rpm, perfect. Do the math and you will find a Muncie is best for racing and street fun. Overdrive trans is best for highway. Before overdrive GM used big first gears with highway rear gears to get a good compromise. That is why the Saginaw came with up to 3:50 to 1 first gears. I have that exact trans in my G-body wagon right now. It has a 3:23 rear, 11.3 to 1 overall ratio and I use it for towing a boat. The 301 engine loves it, but a 400 will work if you don't abuse it. Lastly, BW ST-10's came with a lot of different ratios. The average guy can not tell the difference between a Muncie and an ST-10 so no one will know.

Awesome info.
Thank you for taking the time to respond as you did.
I really appreciate the advice and the reasoning why.

So .... no Muncie for me. I want to keep my 3.08 gears and don't want to change out carrier, gears, etc. for lower "Muncie appropriate" gears. When it comes down to it, this car is babied by me. Never doing burnouts, just love driving her, both in town and on short highway runs where she does 65 easily at this point with the TH350. But I would enjoy a snappier start, and certainly want a 4 speed.

I'm fine with a Saginaw, despite the bad antidotal stories of them blowing up with abuse. I don't plan to beat it beyond the rare "chirp" from the tires. On a retired budget, I simply don't have enough cash to break stuff intentionally and I prefer the rubber on my expensive tires, not the pavement behind me !

Saginaw (411) 4 speed with 3.50 first gear gets me 10.78:1 ratio with my 3.08 gears (I found a nice one rebuilt)
Saginaw 4 speed with 3.11 first gear gets me 9.58:1 ratio with my 3.08 gears (all that I can find are questionable)
I know the Pontiac 400 has more torque than the Saginaw was designed, but I'm not going to be seeing such high torque in lower RPMs with my driving style.

Thoughts ???

Thank you again.

Don't go too low with your first gear. I have a 2.95 first gear with a 3.08 rear and it seems perfect to me. I read somewhere that 9 is the magic number (low gear x rear ratio) and this combo comes in at 9.086.
My driving "style" sounds exactly like yours and when I made the swap I went with a t-5 out of a third gen Firebird. People were quick to mention that they could "blow it up without even trying" and that was ten years ago. It is still hanging in there just fine. At 75 on the Interstate it is happy as a clam at a little over 2k rpms.

I will admit I am biased to a very low first gear and that may not be applicable to your situation. I live in Shitty City and bumper-to-bumper traffic is the norm. I like to be able to just work the clutch in and out and creep along without riding it. If you live out in the country you can probably get away with a less steep ratio. It is up to you. But at least you are researching the issue and that is the key to success. I know the Saginaw gets a bad rap but I have been using them for almost 50 years and I have never broken one. They have a stiff cast iron case and a wide range of ratios. The 3:11 version you mentioned I used behind a strong 400 with 3:23 rear gears for over 10 years. It was a perfect set up with a 10 to 1 ratio. Not a peep out of it and I was not easy on it. Not teenage stupid, but not easy by any means. In my opinion, what kills Saginaws is uncalled-for abuse, and not having the right gears out back. If you take a car with automatic rear gears, like 2:56, 2:78, even 3:08, and force them to work with a manual trans without enough first gear, you are straining the parts and something has to give. It's like riding a 10 speed bike. Do you start out in 5th gear, or first gear? Same affect. The only part I have ever seen fail was the wavy thrust washer at the rear of the main case. They make a Torrington roller bearing to upgrade that, but as I said, with the right gears, no problems. Also, oil leaks kill manual transmissions. Lots of places to leak from, no one checks the oil level. and the first thing to go is the cluster gear bearings. It is not hard to re-build a manual trans and change all the seals. Getting it out to work on- that is the hard part. EDIT: The Pontiac 400 never came with a Saginaw, but the 350 did. Same stroke, same rod length, similar torque. And a factory warranty.

Thanks Olds and Hank !
I appreciate the help.

My dad had a 4 bay repair shop with a machine shop out back. I used to wait for the school bus there for a decade and turned valves on a lathe before school, or replaced brushes on parts that we not throw-away ! I always had oil stains ...
He ran that place from 1950 to 1980.
I wish I had learned more from him when I was younger, before life got in the way, as his fund of knowledge and hands-on experience was impressive.
Came back to cars after his death.

The virtual conversations and tips provided by folks like yourselves do more than you know, and the those sweet memories come back strong.
I'm sure I'm not alone ..... thank you.

I guess we all owe a lot to our Dads. Mine forced me to help him as he struggled to keep crappy VW bugs alive. Besides learning a lot of new curse words, I learned to appreciate and care for my tools. I also learned that I could do as good if not better work myself rather than paying someone to do repairs and maintenance. I miss not being able to ask his advice. I enjoy seeing what other guys have done with these old birds and I try and help when I can. I have been playing with Firebirds for almost 50 years and I have never been without one in all that time. I just turned 70 and my 1968 Firebird convertible will probably be my last big project. I just hope I can finish it before my body gives up!

Look up Autogear in Syracuse NY. They sell all new muncies with different gear sets. 4th gear is always 1:1. But you can get a 2.98 first gear which will be perfect with the 3.08 rear and it will be good highway cruising.

Summary ... car is finished summer '24 !

Picked up a completely rebuilt 4 speed Saginaw from "Old School 4 Speeds" in Vermont. Dave is a great guy who has his own shop (I visited it and he had probably 12 newly rebuilt Muncies, T-10s, and Saginaws) at his home. He worked at dealership in Springfield rebuilding these same transmissions back '60s through 80s when they were covered under warranty, no questions asked ! A wealth of drivetrain knowledge. Also picked up correct original Saginaw linkage and competition shifter for Firebird from him, all adjusted ahead of time and assembled. Also got original refurbished bell housing from him with clutch fork assembled, new factory paint.
His stock on hand is amazing. Shifter assemblies for most everything with linkages.

Followed Oldslow's advice and picked up a forged steel flywheel and upgraded Ram clutch from Summit. Went with his recommendation for 3.11 first gear ratio with my 3.08 rear.
Also put in a new Yukon LSD with new Summit axles - old axles were shot with wear.
(My rear end is GM, not BOP, so Yukon posi fit - easy install, backlash, shimming, etc)

Some issues with new z-bar linkage kit covered in separate thread that Big Chief and Oldslow helped me with. Arm needed to be cut, shortened, re-welded to clear master cylinder. I think this was due (?) to past frame rebuild (rust) and new motor mounts ... needed 1/8" less for clearance. Not sure ... but it worked !

Car gearing is beyond awesome. Oldslow was dead on. 4 speed is smooth, takes off like it has 4.11 rear, and cruises at 70 mph @ 2400rpm. I thought second gear would come on too fast with the 3.11 first, but it is perfect. I baby her, so am hoping the Saginaw holds up behind the 400's torque. Dave at "Old School 4 Speeds" said he put many Saginaws behind 400s back in the day with highway gears. He said "Don't beat it and she will last for decades" ..... we will see !

Thank you all for great advice and walking me through the manual conversion.
Could not have done it without this forum.