Good Dawg.... I'd "kill" for a stick shift in my bus.
Sure, the Allison products are impressively good, but when an automatic fails, it often fails all at once, leaving you stranded. With a stick shift, you can usually make it home. That's a huge difference to my mind.
As for those two Allisons, the MT643 is far preferable, but it would probably come off a diesel engine and would have to be reconfigured to work with the RPM range of your gasoline engine. And there may or may not be such a thing as a bell housing to mate it to your engine.
A Chevy Turbo-Hydramatic 400, built for RV use, might work.
If you must have an automatic, consider swapping in a diesel engine with automatic all in one unit. Big job, though. Reason I'm thinking this, is that your little gasoline engine is probably allready quite worn from pulling such a large vehicle.
Best of luck!