What tranny is my best choice?

Factory Overdrive transmissions are either A500/40RH/42RH which is based on the A904 or the larger capacity A518/A618/46RH/47RH based on the A727. These transmissions were installed in vans, and trucks from 1988-1994 After 1994 they changed to "RE" designating a computer controlled valvebody which are not as swap friendly. Plenty of threads on this and other Mopar sites detailing swapping these into older cars.