A500/42rh

Pre-1994. That is, you want a full hydraulic valve body. Beyond that, cores are pretty similar. The basic transmission is a 904, so all the typical 904 performance mods apply. Also, because these transmissions generally came behind a 3.9 V6 all the typical 904 performance mods are REQUIRED. Not shocking news. I wouldn't bolt a 120K mile transmission behind a stout 360 and expect it to put up with a lot of abuse (although you might be surprised).

A high stall, heavy duty 904 lockup converter will also cost some $$$ as it will be a custom item. Good convertors cost no matter which transmission you chose, but it will be a requirement here. Not a lot of cheap "so-so" converters unlike a 727.

The overdrive makes the tail shaft section of the transmission considerably bulkier, and it will not fit into an A-body rear cross member. Therefore some modification and fabrication will be required for a quality swap. Again, nothing cosmic, but expect to spend a few hundred bucks if you farm it out.

I plan the same swap and have a core waiting to be rebuilt. I contacted SMR for their recommendations based on my intended package (moderate 318, 3200# car, 4.10 gears) with some room to grow (or a +150 nitrous shot). They recommended their 10 3/4" 3000 RPM flash stall, lockup Pro Sportsman converter http://smrtrans.tripod.com/smrtransmissionsintro/id37.html About $600. For the transmission, they recommended their Pro Sportsman rebuild kit $540 http://smrtrans.tripod.com/smrtransmissionsintro/id10.html and Valve Body Upgrade Kit $200 http://smrtrans.tripod.com/smrtransmissionsintro/id14.html

So, you can get $2000 tied up in this swap without trying too hard. Perhaps not out of line IF you are planning to rebuild a 904 with a custom converter anyhow, but something to consider if you'd be pulling an otherwise healthy transmission in search of a couple MPG. Sure, it can probably be done cheaper, but you'll have to determine what is an acceptable compromise.