Whatever you do, please don't trash the HydraMatic that's in it now if you decide to do the swap. They're very scarce, and if you've got one in working condition, or even all the parts, somebody out here will probably want to recycle it. You've also got a lot of unique accessories like crossmembers, brackets, starter motor, bellhousing adapter, and linkages that need to be swapped and saved if the assembly is going to be reused by someone else. Even the shape of the hole in the cab floorboard and the cover plate for it is different on the HydraMatic trucks.
Jerry
Edit: If you decide to rebuild what you've got, FATSCO transmission parts specializes in rebuilding kits and hard parts for obsolete automatic transmissions.
http://www.fatsco.com/index.htmJerry