Strange Pilot/ hub size on 8.75 Rear drum brakes

BBP= big bolt pattern, the 5x4.5" pattern used on pretty much every other Mopar car of the era and most of the later A-bodies, 73+

SBP= small bolt pattern, the 5x4" pattern used almost exclusively on A bodies before 1973 (and some /6, front drum equipped A bodies after '73 too)

As for the drums, you don't need to junk them. What you have are BBP drums, otherwise they wouldn't fit over the wheel studs. And that's why the center register is larger on those drums too. They should be 10x2.5" shoes right? So, those will work with BBP axles as long as the axles have the normal BBP flange offset. The axles you have use the larger bolt pattern, but have the SBP axle flange offset, so the axles must use redrilled SBP drums (10x1.75"). So if you get standard big bolt pattern axles, you can keep those drums. If you keep the axles, you'll have to find a disk kit for small bolt pattern axles, because the ones intended for big bolt pattern axles won't have the right offset either.