Backing plate offset

from the ad:

"Part # A30-CST-A4- A-Body, 8 3/4" Rear, Stock A-Body Drum Brake -- 30 Spline"

With that description I'm going to say you'll have to use the A body 8-3/4 backing plates.



So to answer your original question:

"My question: can I use the backing plates which come from the Dusters original 7,25" rear end? Or can I use the units which came with the B-Body rear end?"

You can not use either set of backing plates. If the original drums from the 7.25" are 10" you can use those but you'll need the 8.75 rear drum backing plates.
I know, it all gets very confusing.