A500

Big drawback to the A500 is the lockup torque converter. 904 lockups have never been known as rugged pieces. I'd bet there may be some decent after market parts, but the factory stuff was sorta weak. Oh, you have to do the same rear crossmember and mount fabrication as the 518.

The 500 does have the lower gear set, which is a good swap, and can be put in a 904 as well.