A-Body 66 Barracuda 8.75 backing plates

Miranthis said

Thanks! I keep seeing a reference on line to a towing package and "some folks say" that meant HD 10 x 2.5 rear brakes. Of course, "some people say" does not control. I have yet to see a document that says the larger ones were in the 1st gen. A-Bodies....

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No, pre-'73 A bodies never got anything larger than 10x1.75.
If I'm not mistaken, on a base model the towing package got you an 8.75 rear (and the 10" brakes by default- so they also put 10" on the front); HD braking could get you 10" brakes on a 7.25 rear (depending on other options) and 10" drums up front, and then there were the optional front disc brakes. There's a lot of possible combinations, but at no time were there ever any rear brakes larger than 10x1.75 prior to 1973.