Backing plate offset

Hi all,

I know there are already several threads regarding backing plate offset. But these just added some confusion:???:
I am currently building a 8,75" rear end for my ´72 Duster. The housing is an early B unit which has been narrowed to A-Body factory specs. I bought SBP axles from Moser (also factory specs, I guess 27,***" length...).
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?

Thanks in advance and greetings from Germany :)
Sebastian