What is a 42RH Transmission?

Basically the A500/42RH is the OD version of the A904 family. Hydraulically controlled as opposed to the 42RE which is electronically controlled. They were used in V6 and 318 Dakotas, vans, and light duty pickups.
The big overdrive housing on the tail of the transmission necessitates substantial floor pan and transmission crossmember modification, as well as fabrication of a new transmission mount in order for it to work in an A body.
42RH still has a speedometer drive that is compatible with the gear and adapter housing from the 904; many used a drive that mounted a VSS, just remove that and install the standard speedo drive. You will most likely need a longer cable since it is located further to the rear on the 42RH.