If you use the 10” SBP backing plates that came with the 8 3/4 you should be able to use all the other brake hardware from the 7.25 10” SBP brakes- drums, shoes, self adjusters etc. The 10” drums with the flare or “bell” was just a design thing, they made them both ways and other than the extra flange the dimensions are the same.
10x2.5” rear drums are all BBP. The axle flange offset is different from the SBP axle flange offset. So, if you want to use 10x2.5” BBP rear brakes later you will need new axles, new backing plates, drums and shoes. The hardware, like the spreader bar and parking brake levers can be reused. But that’s it.
