The actual installation is probably the hardest part of this swap.
The A500 as mentioned is a 904 with the same OD section as the A518 and there are lots of good HD parts available for the 904. The big bug-a-boo is the overdrive planetary setup. The early design had a 22.5 degree tooth pitch and had a lot of failures. The later design went to a 15 degree pitch and the problems went away. You can get an overdrive geartrain kit with the later goodies in it from Transtar or WIT. Be awful damn careful when you take the OD section apart. There's a spring in there that is under tremendous pressure and if you don't have the tool to take it out, you could be missing fingers or worse.
As others have said, a high stall not a big deal, and to keep things simple, get the early (1988-1996) hydraulic trans.