Did you want to go with a 7" rim, or was that part of getting it to fit with that backspace?
Either way, if I really wanted to run 15x whatever stock rims, I don't think I'd go the B body rear end route. By the time you've bought the B rear, moved the perches, and had custom backspace rims made up, you probably could have bought an A body rear end.
Now, if you want to throw a set of 17 or 18" Mustang Bullit rims on there, the B rear is the way to go...