An accurate 8 3/4" rear axle width list

Yes and no. Width matters so pay attention. Backing plates usually interchange. Though Your best bet is to get a back plate drum set.

No the 7.25 mounting plates will not work in your new 8.75. 2.25 inch diameter tube versus 3 inch. I tried to hog my plates out but the shock mount itself hit the axle.
Will 8 1/4 backing plates work on the 8 3/4?