Sorry, don't have any real numbers. But, here's what you go by, without parts numbers for the backing plates. The one constant in it, is the measurement from the axle housing flange, to the drum mounting surface, at the axle flange (outer surface) itself.
So...if you lay the drum flat, measure from the inside of the mounting pad, to get the width of the drum. If it's a 3", should be a little wider, for shoe clearance. For the backing plate, the offset measurement will be even more, since the inside edge of the drum needs clearance, to the edge of the backing plate.
Differences will be about 1/2", since drums are 2", 2 1/2", or 3". Each width needs same drum to plate clearance. That's why the different offsets.