OD tranny hard to come by?

Technically the GV gives you 6 speeds but for practical purposes it's just an OD that you engage when the tranny is in top gear. No one is going to manually shift an auto by running it through 1st gear then engage the GV, then shift to 2nd and disengage the GV, runn it through second then engage the GV, etc. I am not even sure that makes sense from a gear ratio standpoint. It could be that you shift 1, 2 in the tranny then shift back to 1 and engage the GV, then go to third with the GV off and then to 2nd with the GV on, etc.

I wouldn't bother with the A500 and would look for the A518/46RH. The A500 was used behind v6 engines and the A518 behind v8. I would think it's going to be easier to come across an A518 because very few RAM pick-ups had v6's. The OD unit is the approximately the same size so the floor tunnel surgery will be the same regardless of A500 or A518. Also the A518 would only require a stock type rebuild where as the A500 will need some beefing up which would keep the cost of the A518 rebuild less than the A500.
FWIW, you can use the early than 90 A500's if you cross drill the main shaft in the OD to fix the oiling problems like the 90+ units have. it's pretty straight forward to do and if you pick up a rebuild kit there will be instructions on how to do it.

Also, there are a number of articles on the web on how to rig a couple of pressure switches to the A500/A518 and a switch to the throttle to engage the OD and lock-up automatically.