I have a swap article that was published by the now gone hi performance mopar magazine about installing an A500 or A518. It's actually 5 articles and is very well written and informative. If you want this article,just p.m me with your email and I will send it to you. At least you will be better informed as to how much work is involved in swapping an overdrive chrysler into your swinger.
As for the company that wants to sell you a g.m tranny,thats exactly what they want to do,sell you stuff. It might be a good swap but Im sure you'll run into snags just like every non stock swap.