How many using an A500 overdrive ?

Since you have an A100, "things are different." The A bodies have a problem with tunnel clearance, and a person (search there are threads) can cut out part of the rear crossmember, re-work it, and shoe-horn an A500 up in there. "We are told" that an A518 requires floor surgery

I don't remember the A100 crossmember.......don't they have an "overhead" hanger setup? Anyway, the tunnel/ crossmember will likely be your problem

I guess? you are aware of the differences re: the earlier RH, and the later RE (electronic?)