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

I don't know much about them but I would expect that the simpler, the better.
I had a 518 a few years back that was a respectable rebuild using Cummins spec internal parts but it was the electronic (46RE ?) version. I didn't want to deal with a stand-alone controller or manual valve body. The lower 1st gear of the 500 is a better idea in my opinion. One could run a taller axle gear and still get good acceleration from a start.