Here are 15x7s with 255/60-15 M/T SS Drag Radials on a 69 dart .
I have 15x8 with same tires on my Cuda and it looks like the overall width of tire doesnt change much.
What back spacing do you have on the 15X7 rims. My 15X7's only have 1/4 inch back spacing and with the 235/70/15 I'm very close to the outside wheel lip, I had 245/60's on it and it was super close to the outside wheel lip. The 15X8's I'm looking at have a 3/4 back spacing which should give me enough room for the 255/70's or 60's haven't decided which one yet I hope.