The difference in the track width can't be more than the thickness of the drum where it sits on the axle flange compared to the thickness of the rotor in the same place.
There is a slight difference in track widths for SBP to BBP, but I know all of my measurements are using a set of BBP axles from Dr. Diff, so that's already taken into account. All that would be left is the rotor thickness compared to the drum.
