Duster has a longer wheelbase. 7 1/4" rear is shorter from axle centerline to u joint centerline than an 8 3/4". Driveshaft won't be the same as a 66 A Body.
Was referencing this thread a few ago where a guy needed a replacement 49.5 driveshaft for a 74 Duster 904/8.25 so they are out there stock length. Wheelbase can be deceiving. rear ends do make a difference, you are correct. I was incorrect when stating it was from a 904/7.25.
