What rear axle is in that '73 Duster? Big bolt pattern?
I had a set of 275/40/17's on my '74 with no modifications. There were on 17x9's, spaced to 4.75" of backspace. They were a little tight on the springs, but nothing rubbed, and there was plenty of clearance to the quarters.
The BBP 7 1/4 and 8 1/4 rear axles are nearly an inch narrower than the earlier 8 3/4 axles, so the backspace needed is a little different. For either of those I would say 4.5" of backspace for an 8" rim, 4.75" for a 9" rim. With an 8 3/4 with BBP axles you'd want more like 5" of backspace.