255-60-15 MT drag radials on my 68 dart

According to the tire calculator, just throwing on the new tire will be almost .5" closer to the fender. You will either need to roll it or do some trimming. Also note that the new tire is about an 1" taller. I have 255/40/18's on my Dart. They're the same size, just an inch shorter. I got wheels that are almost perfectly centered in the wheel well and I have some slight rub on the fender side. And that's with about 1/4" or less clearance to the leaf springs.