Is this for that '71 RoadRunner?
Because if it is you could probably put 295/50/15's on 15x10's on all 4 corners and still have room. I slung the 17x9's with 275/40/17's I had on my Challenger on my '71 Satellite just to check the fit and still there was still plenty of room everywhere. As in, inches of extra space. It would be a piece of cake in the back, the width would be fine in the front but the height is getting pretty tall.
You could literally run 15x10's with 5.5" of backspace front and rear. Front might need a small spacer, but probably not. With the stock 10.95" BBP disk brakes up front and the stock '71 B body 8.25" in the back with the stock spring locations I figured you could run a 10" wide wheel up front with a 5" to 5.5" backspace depending on your tire choice, and a 10" wide wheel in the back with a 5.5" to 6" backspace. 15x8's up front would be a piece of cake, the biggest problem would be finding a nice wide tire that wasn't too tall. That's why going to a larger diameter rim is nice, because you can get 255's and 275's in a height close to 26". Even on the 71 B's I don't think you'd get away with a 27" tall front tire unless the ride height was super high. Because of the tire choice you basically have to either run a 245/60/15 up front or a 295/50/15 to get the height in the ball park. Or smaller go narrower than a 245/60/15, but anything smaller than that is gonna look tiny on those cavernous '71 B body wheel wells.