B body rear axle in an A body

The 65-67 B body rear only buys you an extra 5/8" on your backspacing compared to the '68-70.

While that would mean you could run a factory 15x7, 4.25" backspace wheel with a 255/60/15, that's the best it'll get you.

Unless, of course, you run the ride height so that your quarter lip is a few inches above your tire. At which point you can run anything you want under 28" tall, because it'll just stick out past the quarter lip and you should have enough clearance that the rubber never hits the body. But that's not actually fitting the wheel inside the wheel well.
I had the 15x7 rallye's, with 235/60r15's. Fit like a glove. However, the aluminum slots in the picture are some deep dished, and I did have to fold the inner lip up to keep them from rubbing. I've always like the 65-67 b bodies in Dusters. In fact, I just set one in my other Duster, still have brakes/driveshaft etc to do.