The "correct" width for a rear axle the one that works with the wheels you want to use. And that depends on a lot of things. Spring location, mini-tubbed or not, 15 rims, 17/18" rims (or bigger), body style- Dart vs Duster, the rim you really want to use that only comes in a really weird offset...
I use a B-body rear axle in my Duster that's 3" WIDER than the A-body axle. And guess what? It's the "correct" width for my car with 18x10's and 295/35/18's.
