Changing rear end bolt pattern.

You can have the axles & drums redrilled easily enough, but while they're at the machine shop remember to have them turn down the center register, too- otherwise your sbp wheels will never seat on the drums.
Edit: When turning down the center register, you may need to "step" it; leaving enough of the original diameter to center your brake drum, and stepping it down after the drum thickness to the wheel register size...