Backing plate offset

Your axle offset choice is what can limit you to using only the 72 and older A body 8.75" rear end 10"x1.75" brake backing plates. The distance the axle extends from the flange it bolts to is what determines the offset you need for the backing plates.
Just redrilling the axles will not solve that problem. The center register will be too small for any mopar brake drums except the A body small bolt pattern drums.

Just to clarify, what exactly did you get for axles? You stated "I bought SBP axles from Moser (also factory specs, I guess 27,***" length...)."
What is the center register diameter? Thats very important.
Any other specs listed for them?