rear brake shoe size

Demon 408 said
From the 1972 factory service manual the standard brake size for a /6 and auto trans: fronts 9 x 2.5, rears 9 x 2.

Unless you have 10" brakes which is quite possible. 9" were the bottom-of-the-barrel brakes and didn't work well at all. 10" were somewhat better and disc brakes were the best.