Backing Plate

Have a 1965 Dart GT that I had to pull the differential out. Saw that the wheel cylinders are leaking and noticed on tear down that the wear pads that the lower part of the brake shoes ride on are looking worn. There is a ridge that you can feel. Is there a way to tell when to replace the backing plates? Can anyone tell me where to I can buy some backing plates?
Brakes are 10 1/2 x 2.
Is a 8 3/4 rear end.
They can be sanded, ground, or repaired. If you are looking for replacements, start a parts wanted thread.