Looking for wheels

With a '71 Dart you should be able to fit a 275 in the back with just a 1/2" spring offset, so the only thing you'd have to do for that is move the axle perches. Getting into a mini-tub and shortening the housing is going to get pricey really quick, so it just depends on how much tire you want and what you want to spend. But if you can live with a 275/60/15 you can use the 1/2" spring offset and 15x8's, lots more options for rims in that size in a backspacing that will work even with the 275's. Doing the 1/2" spring offset will cost a couple hundred bucks for the hangers, shackles and perches, plus whatever you have to pay a welder to move the perches if you can't do it yourself. The spring relocation and mini-tub will cost close to a couple grand if you pay someone to do it, and then shortening the axle will be in addition to that if you do it. Unless you're pushing really big horsepower 275's are plenty of tire.