What are the most likely Donor vehicles for the A 500 4 speed automatic?

Jackpot! I think.

Further reading of Gary's documentation shows that the 42RE was actually introduced in '93 for the GC 4.0 inline six and came with a stand alone TCM.

View attachment 1716149897

While I'm not suggesting someone try and use the transmission, it does mean there is a controller for an RE transmission. Probably still needs TPS.

And it would be JY prices.

Here is the TCM pinout:

42RE-TCM.jpg