10 X 2-1/2" rear brakes on my /6 Duster?

Yes, on '73 and up A bodies when you got front disc brakes (power or manual) you also got 10x2 1/2 rear drums and 5x4.5" bolt circle all the way around, regardless of which rear end you have (7 1/4 or 8 1/4).
Some 8 1/4s even got 11 inch brakes, at a time when the assembly line was "using what they had" to keep the line moving. I've got one of those- you'll never find that one in the parts books.