70 duster 8 3/4 conversion to 4.5 drum or disk hardware

A body stock 8-3/4 backing plate will ONLY work with stock offset A body axles. The A body 8-3/4 brakes are unique to ONLY the A body 8-3/4. No other MOPAR rear (post 1965) uses the same backing plate.
If your going with new axles AND large bolt pattern it would be wise to replace the entire rear brake assemblies.
I couldnt get your link to open.
Rear drum brake assemblies from any 1965 up 5 on 4-1/2" bolt pattern will work. The later 7-1/4 and 8-1/4 10"x 2-1/2" brakes work excellent. I have a couple complete sets of those with good drums.
I would use the axle set Doctor Diff (Cass) sells, he knows his stuff.