hey guys,

blew my 3.08 diff in my 68 bird the other day (thank god for that!) - got some 3.55 gears to put in place, have a powerglide behind the 350, but wanna get the bird to move off the lights a little quicker. 350 has a holley 600, some headers, torker manifold (probably too big) mild cam..
my dilemma is - do I keep the "glide" or go for a T350 or T400....and will the turbo350 or 400 make the car accelerate quicker??

Yes, because you will get a real 1st gear. I had the ST300 and a TH350 made a big difference for my Sprint.

Your 3.55 gears will make a difference and adding a TH-350/400 will also make a big difference. I have 3.73 in mine and had a TH400 and the gears alone made the car move much quicker off the line, but highway speeds were almost unbearable since i was running about 3100 RPM's at arouns 60/65 MPH. I put in a 2004R overdrive tranny and the gearing is a lot better. The car launches faster off the line now and highway speeds are down about a 1000 RPM's from what they were with the TH400. I have copied and pasted some gearing info below so you can have an idea of what the gearing difference is between trannys and help you decide what way you want to go.
GM Automatic Transmission Gear Ratios
Transmission
_Model----------First Second Third Fourth
Powerglide.....1.76 or 1.82 1.00
TH-200-4R......2.74 1.57 1.00 0.67
TH-350...........2.52 1.52 1.00
TH-400 ..........2.48 1.48 1.00

Here is a link that shows different gtranny gearing and right below that is a chart that will show you the mechanical advantage your car will
have coming off the line - higher number is better (up to a point) with each tranny and based on your rear end gearing.
http://www.race-track.com/news.asp?TOPIC_ID=532

...and here is my calculator for expected RPMS, etc. based on transmission, rear-end gear,etc.:

http://groupwhere.net/firebird/speedo.php

yeah, know what you mean bout the car screaming at 60mph - had 3.91 gears before with the glide and had to turn the stereo up full blast to hide the engine screams!! looks like will go with the T400 as my mechanic has a rebuilt BOP sitting there....thanks guys for the posts - really helped heaps....

Changing the rear gear will give you a big increase off the line. I'd wait until I tried them before starting to think about a tranny swap. Those powerglides are very popular with drag racers. If you do a lot of highway driving then an overdrive is the way to go.
I'd say changing the intake manifold will make more difference. That torker manifold operates from 3500 rpm to 7000 rpm. It's called a toker but that's just the name. You will get better off line power and mileage with a stock manifold, a performer or a performer rpm. I think the performer would be best suited to your car, it operates from idle to 5500 rpm and is a stock bolt on.
My opinion anyway.

I think you guys are nutz. I would stick with the 3:08s and replace the tranny to the th350.

I have 3:36 rears and they are still to low for highway driving IMO. I have a 8.5 3:08 waiting to go in.

When I open this, parts are overlayed. Anyone else able to open it successfully?

Its overlapped for me too.

Yeah, that's crazy. Looks ok except in IE... Working on it.

Use the compatability view to make it work. It was doing the same for me and I went to "TOOL" and selected Compatability view settings and added the site in there and it works fine now. I am using IE 11 so if using an older IE it may be listed in the tools differently.

You may be correct rohrt, we may be nutz, In my case almost certainly. But matigas said he wanted to get off the line quicker, he didn't mention anything about the highway.
If I was going for a compromise I'd go for a 3.36-3.41 and keep the powerglide but definitely change the manifold.
But that's just me.

Should look better anyway now. I went to the original site and it was also messed up there.

I say that jokingly of course. It may be my age or just getting to use to driving more modern vehicles but being able to cruis down the road hiway at 2200 RPM is a joy to me. Both in terms of engine noise and MPG. In our old cars trying to have a decent option for a transmision with 4 or more gears is a expnsive endeavor. There are of course varying opinions on gear compromise. I loved my 77 GP when it had 3:23 gears. I didn't care for the 3:42 I put in years later or the 3:73 gears my brother put in his TA that also had 3:23 previously. I have a 455 in my FB as I mentioned with the 3:36 gear and trying to hold it at 55-60mph is no different then putting it Neutral and and trying to hold the RPM at 2500 and the 2.5 droning exhaust is just annoying.

My Tempest I bought two years ago with the 2:56 with the 326 and 2 speed has been something different for me to experience and I just love it. It can hardly get out of its own way but that is another story. Its still the one I can just jump in anytime and drive.

The car weight, motor size, cam and converter all play a part in whether it all works well. But since the OP already has the 3:55 gears it sounds like he knows what he wants. I would say to the OP go with the th350. It should slid right in without any changes to the cooling lines, converter, cross member or driveshaft. You will need to a linkage connection for the kick down but that should be it.

Since you say you already have the 3:55 did you buy a new center section? in mind has he bought a new center section?

There were 3 different ratio 8.2 Pontiac 10bolt carriers for both single track & Safe-T-Track usage
High ratio = 2.56-2.78
Mid ratio = 2.93-3.08-3.23
Low ratio = 3.36-3.55-3.90-4.11-4.33-4.56-4.88 Eaton posi for the 8.2 only in Low Ratio

man, I go away for a week and so many options....well, going with the 3.55s with a new LSD carrier and the turbo 400 - car mainly driven around the city streets - frequent short and quick trips to get from a to b and c, so yeah want some better acceleration. totally agree that I have the wrong manifold - needed one desparately one day and that was what I had spare,,,,will definitely go back to a stock manifold and quaddie carb soon though....car back on the road on sunday with the upgrades (or downgrades for some, lol), so will post verdict next week....
UPDATE: pulled the "powerglide" out and it's one of those Super Turbine 300 two speeds - made for olds/buick and pontiacs from 64 to 69 - anyone had these - I guess there are a few around....
VERDICT: on the positive side - drives like a totally different car - much quicker all round, off the mark is now nearly twice as quick as before, car feels so much lighter and easier to handle....
Negatives....much noisier at 60mph than before...about 2800RPM against 2200RPM, MPG is a little worse...
I don't do much highway driving so noise won't bother me yet!! and anyway, national speed limits here in OZ are mainly 60mph...