[Lugnut64052]

4X4s had HD cases. Not sure if they all did, but some did for sure. The HDs did have an aluminum flywheel cover with a couple of brace rods hooking to that cover and going up to the engine.

There's a huge wealth of aftermarket and good used parts available for Th400s and you can build (or have built) whatever you want. The heavy duty version was called a Th475 and basically the only difference was that it had straight cut planetaries, which was supposedly stronger, but also noisier. There have been several clutch piston styles and depths over the years. Bigger clutch stacks were usually done by using lower profile pistons. There are aftermarket, deeper drums available too. I expect there were quite a few different clutch stack setups from the factory since they made them for 40 years and put them in virtually everything.

Converters from the small Allisons will fit the 400. The basic difference is the Allison converter has steel internals vs. aluminum internals for the 400. There may a stall speed difference too (not sure about that though). And as mentioned, the Allison converter has six bolt pads, vs. three for the 400. I don't *think* any Allison converters were factory fitted to Th400s.

Some heavy duty ones did not have a parking pawl. My dad owned an Airstream motorhome so equipped (or non-equipped) that way once. It was a one-ton van chassis. It also had a mechanical cable installed in place of the vacuum modulator.
BTW, the parking pawl is a small hook inside the tranny that latches into splines on the output drum, providing the "Park" shift position. Allisons don't have the park pawl, and you have to put them in neutral and set the parking brake to park them.