Which OD Trans

I think the A500 is a pretty good choice but here are some of the details some don't realize.
1. Obviously the cross member.
2. 3/8 cooler lines from the A500 instead of 5 1/16 (best to use new 3/8 lines and change fittings at the radiator)
3. Longer speedo cable (the 80 inch works well)
4. Need to run relays for the OD and LU
5. Trans needs fused ignition 12v for the solenoids.
6. Driveline will need shortened about 3.5 inches.
7. Trans angle for the driveline angle will likely require some messing with after.
8. Being the A500 in commonly a truck trans it already has the low first gear.

My A500 uses adjustable pressure switches plumbed with cooler line to the governor port for OD and lockup so it shifts fully automatic if the switches on the shifter are both on. (OD at 50mph and lockup at 70) and of course it comes out of those two at those speeds because of the adjustable pressure switches.
With both off it won't use OD or lockup at all, and an adjustable vacuum switch for kicking it out of lockup, OD or both when intake vacuum gets low enough to trigger the adjustable vac switch.
It's all in how you wire it and what parts you use, but in the end it's worth every bit of what it takes to do it.