8 3/4 Backing Plates

I have a 70 Duster with a 7 1/4 rear. I got an 8 3/4 housing, also with the small bolt pattern axles. I want to get the large bolt pattern axles from Mancini Racing. I know the brake drums are different, but, Do I have to change the Backing Plates of the 8 3/4 rear if I go to the large bolt pattern axle????
I will still keep the 10" brakes.
Thanks
Pete