My 68 convertible has the th400 in it. Added fluid when I got it and it seemed real finicky, too much fluid and it would go through the different gears without really engaging. Too little and you could feel it engage but kind of felt like it was slipping. Rpms would go up and finally it would start moving. Noticed it was leaking and replaced the tank seal and vacuum modulator. Seems like I fixed the leak but it still doesn't engage very well. Doesn't make any odd noises, just will build rpms when you try to take off then will catch and go but not with any real conviction. Where would I even start to solve this conundrumI?

Have you changed the filter? I got stranded once, it acted the same, filter was full of crud.

Ya, put a new one in when I replaced the seal

Likely needs a rebuild or replacement.

It could be several things then. Slipping torque converter, worn out clutches, bad pump, internal hydraulic leaks, a lot of things. But there is hope. A GOOD transmission shop can hook up gauges and see what the line pressures are. There is a plugged port on the side that a pressure gauge screws into and they can test for line pressure. A T-400 rebuild is not cheap but if you go for the best quality parts it will probably outlive you. A cheaper T-350 will bolt in but needs a different cross member and drive shaft. With a Pontiac 400 I would re-build the T-400. I once re-built a T-400 in my garage with basic tools. It was difficult but not impossible. My mistake was adding a shift kit that made the tires chirp every time it shifted. Lots of fun, but I forget that by boosting the line pressure, it needed an extended warm-up time in winter. So one day, running late for work, I blasted off only to hear destruction going on in the tranny. Live and learn.

Ya, that's always my fear. Tackle a project that's a little to big, get it pretty darn close but that difference ends up costing more than having a pro do it. We'll see what I've screwed up so far when I finally get it on the road and put the hammer down a few times. But on the plus side I have roadside assistance with 2 different companies, so up to 8 free tows.

When my kids were in college, I gave each a 2nd gen Firebird to commute with. One was a 1979 Formula with 301 and T-350 trans. Since it had 150k on it, I had the 301 and T-350 re-built. The 301 was re-built by a real engine shop that does professional work. The T-350 I used a local tranny guy and he said there is a $200 rebuild, and an $800 rebuild- which do you want? I went for the $800 rebuild because I already had a $200 rebuild fail after two years. That was over 20 years ago. That T-350 is so solid that it is still being used in a 1980 Trans Am w/400 right now. It still works perfect. The 301 is still powering my 1983 G-body wagon. You get what you pay for. Since they haven't made a new T-400 in decades, it would be unusual to find another original like yours. Most will have been re-built, often several times. Once diagnosed, if your T-400 needs a re-build, I would just park it, save up, find a competent shop, and consider it an investment. You will never get stranded or have to worry about it again. And if you ever swap over to a manual trans, you can re-sell the T-400 easily.

$800 rebuilds are a thing of the past.
any transmission that has been sitting for years or you don't know the history most likely needs a rebuild. Anybody that can pay good attention to details and has some tools and mechanical skills can rebuild a T350-T400. Just lay it all out on a table in the order you take it apart and put the lip seals in the correct way and also make sure the spool valves are not stuck or binding up in the valve body.
Air check the forward and reverse pistons when you get the assembly together before you assemble the whole thing.
Piece of cake

Got a recommendation on a particular rebuild kit or are they all about the same?

You are going to get what you pay for. Stay away from the el cheapo kits. Lots of choices, Jegs, TCI, but B&M is probably the best. That is what I used. Just be sure it is a COMPLETE kit. Are you going for it? Read the FSM, and any other T-400 specific books you can find. I needed three different manuals to cover it all, each seemed like it left some stuff out. I did it with basic hand tools and feeler gauges, but I as poor then. Now I have better tools. And of course, workplace cleanliness is the most important thing. And when you remove the valve body, take note of the position of all the check balls and write it down, or you will absolutely forget where they go.

I'm still undecided, I've got the room to do it. Right now I am recovering from surgery after tearing my bicep tendon. But it may give me a good winter project.