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.

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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.
I've read references to this before--that certain jeeps had stand alone controllers and allegedly can be used for an RE swap into something else