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

Interesting. The documentation from Gary in the link above says the 46RH was introduced in 1990 and did not have LU. And then in '91, LU was added.

So a '90 46RH looks to be your huckleberry if you want OD but do not want LU.
My 91 A518 was a non-LU. It's in my friend's Cuda and I now regret letting it go.