Hard shifts /lockup???????
I have an MT643 in Millicent, and she shifts like a steady flow of chocolate martinis. Now... that AT545 I had in my first bus (RIP), that thing shifted like a #$%^&* sledge hammer!
It's an old but true Thing about automatic transmissions, that they have individual personalities. Look around on the walls of any transmission shop, and you'll find faded magazine clippings and faxes about the ten different ways an overhaul will turn out -- of the exact same transmission model.
The MT 643 is a far superior transmission -- that much is certain.
Among buses and trucks, engines and transmissions can be swapped almost without limit -- all it takes is the correct bellhousing for the match.
The driveshaft is no problem. Look in the Yellow Pages under Drive Line and Drive Shaft Service. A local such shop can mix and match anything.
Time for another chocolate martini.