for production ease, i would assume.., but i am only assuming here, that the rears would be the same with... drum to drum, and any slack they have - from larger brakes would be made up " behind" the drum to drum measurement..
if you convert to large bolt pattern on a 8 3/4 rear the rear will be a little wider.. i forget how much but it really isn't much.. not sure on an 8 1/4 rear though...