They are a lot stronger than given credit for. Even though they were only used behind the V6 they are a heavy duty version of a 904. As for the O.D. unit it can be beefed up as well. The A500 would be slightly easier to fit in an A-body since it's shorter. The big problem is the large diameter of the O.D. unit which is the same on an A500 and A518.
Babyblue66 I'd contact Chris Andrews at Andrews racing and see what he recommends. I'm not well enough versed to give much advice on the A500 except it isn't as weak as some say. Heck for a long time people claimed the A518 was junk and only good enough to use behind a mild small block. I know a guy running an A518 behind a Hemi making in excess of 800+ hp in a heavy Superbird. Just goes to show how things change.
Here's a link to Andrews Racing's website.
http://www.andrewsracingtrans.com/Transmissions/727/index.html