A-Body 66 Barracuda 8.75 backing plates

Miranthis said
so are the original rear drum brakes on the 8.75 rear 10 x 1.75 or the larger 10 x 2.5?

As I stated previously, the only brake to ever come from the factory on an A body 8 3/4 rear was the 10x1.75 drum brake. Regardless of year.