I'm right in the middle of the same swap; I went with 10"x2 1/2" drum brakes that came out of a 70 Challenger. The 340 Dusters originally came with 1 3/4" wide brakes, the 2 1/2" brakes from any E or B body will fit (10" / 8 3/4 rear end). Had to get new (longer - 28") axles but was switching to BBP anyway so not a problem.