B-Body 8 3/4 Rear End in an A-Body

The B rear swap is much better solution for a Duster or even a Barracuda than a Dart/Scamp/Valiant. The Duster's larger wheelhouses make it a much easier fit, even with the increased backspace rims. The stock A body width matches the Darts much better, its actually kind of short for the Duster.

If you did do a B rear swap on a Dart the '65-67 B would be a better choice than the 68-70. The width on the 68-70, and the backspace needed on the rims would make changing the wheels a pain in the butt.

I'm going to swap a '68-70 B rear onto my Duster so I can run modern 17 or 18" rims and run the same backspacing as up front, but I wouldn't do it on my Darts.