I bought the drums off of Amazon,and they fit the center register and bolt pattern fine. They did contact the backing plate, so I had a friend machine off 1/8" from the edge and the small flange and have plenty of clearance now.
Sounds like the ones you received were intended for the front, but removing the "snow lip" made them fit like I suspected (post above). If so, easier if you get ones with 1 lip.