08 legacy rear drum problems

Doing new rear rotors and pads. new rotor is bottoming out on a rusty backing plate. Wheel would not turn upon assembly when lugs were tightened.  Removed caliper and retainer and used washers to shim lug nuts and tightened. Drum locked up. Tried old one and it was fine and spun when tightened.

  Seems to be a rust problem on the backing plates so I have tried a chisel to knock to plate back. This is not the outer backing plate but a smaller thicker inner ring and it fits into a groove in the drum.

trying to find any info? should I try to replace backing plates? Grind and pound ring in? appears to have loosened a bit after chisel adjustment..thanks for your advice..Paul