long tail shaft or short? That's really all the differences you need to worry about per se, since a TH400 can be built up to do whatever you want it to do. If you have a short tail shaft and matching housing, you can swap that into a long tailshaft unit if need be when rebuilding. That's one of the things that's cool about the 400 IMO, save for the insane amount of torque you can try to punish it with. It's a strong transmission right out of the box.
If you're rebuilding it this is a moot point but not all created equal, the one that came out of a '72 Skylark behind a Buick 455 I would assumed would come with a 34 element sprag but it had a 16 element sprag.
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