will 275/60/15 fit 71 Duster ?

On a Duster, won't work on a Dart. Also, you're car is sitting up pretty high in the back, so that makes the clearance on the quarter less important.

I have 295's on mine as well, but mine are 295/35/18's. With the car lower I trimmed the quarter lips to keep things from rubbing. Even mine sits up a little higher than I would like, but the springs don't have many miles on them so they may settle in a bit still.

No rubbing on the inside?
I have ss springs, with "quick" turns i have a gentle rub on the inside. The lower the car is the bigger chance for this to happen with body roll. Gas dampers made a change but not that much.
I will trim quarter lips and add spacers to make this go away. Also wanted the car to sit lower but I will do it only after I put spacers with longer studs. I made the front sit a bit higher so overall it looks better for now. Also I have deep pan and wanted to have a better clearance.