what do you think is the best stance for a 71 duster?

Build your car exactly how you want it.

If the car is built right, stance, style, brakes, suspension, A/C, overdrive, modern electronics, etc........tire/wheel size will be secondary to the overall look

If you are putting in a 6.1 hemi and a paddle shifter Manumatic, 15" wheels would be silly and not fit the theme.

The Demon in primer on top is gonna have a 345 wide rear and a 255 front with a 1.5" rake. The real wheels are on order for that car, should be here in a month or so.