8 3/4 width after converting to 4.5x5 and 10x2.5" brakes?

The overall width will go from 57.125 with SBP brakes to 57.9375ish with bbp brakes.

Doesn't matter if you use 2.5 or 3" wide brakes, overall will be same. Backing plate offset takes up the extra 1/2" of shoe width.