drum brakes and moser axles

The 1 3/4" drums have a small "register" that the drums center on. It's the round, raised area in the middle. As mentioned, the 1 3/4" drum/backing plate assembly has a particular offset, which works with the axle bearing position that is unique to small bolt pattern A-body 8 3/4" axles. 2x10 backing plates and drums won't work if the axle bearing location is cut for the original, smaller brake assemblies. So it sounds like you have to use the smaller brakes and redrill the OEM drums for 4.5". Not sure how easy it is to find new drums when needed.