[mechanicalman]

I see no reason whatsoever to put the TH400 in. It would require the mods already mentioned, and there is no guarantee that it is in better shape than the trans you have.

Torque converter will interchange if you have a dual pattern flywheel. Most flywheels are dual pattern, but a few older ones only had one pattern. I'd prefer to use the TH350 torque converter regardless which trans you go with. But I'd go with the TH350 and it's own torque converter. The TH400 requires more horsepower to run than the TH350, the internals are heavier and more mass to get moving (lots of drag). TH400 will withstand more power, but I doubt you will come close to exceeding the TH350's power limits.

You might consider replacing the torque converter to front pump seal while the engine is out.