The spacing between the axle/backing plate mounting face and the wheel mounting face on the axle is different between small and large bolt pattern. In order to run any of the commercially available rear disk brake kits, you would need to have the correct offset (or make your own modifications). In the 7 1/4, you still have to swap the whole axle to do this since apparently the axle shafts themselves aren't the same length.
So this would mean also that the drums are different widths. The small bolt stuff is kind of it's own game and doesn't have a lot of support.
