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

As long as you get your new 5 on 4-1/2" axles set up for the standard Mopar offset, NOT the A body 8-3/4 offset, you can use a lot of different rear brakes.
I picked up a set of axles for my Demon's 8-3/4 from DoctorDiff (highly reccomended)and used a set of 11"x2.5" from a 74 Satellite Sebring Plus I parted. The 11+" discs from the same car are on the front.
I have a couple sets of 10" x 2-1/2" rear brake assemblies available that will work for you, if your interested.