That's certainly not the norm. It does depend on the rim design though. Were those mustang Bullits or a different style?
For the 275 it doesn't change all that much, a 6" backspace is pretty much the minimum on a 9" wide rim to get a 275 to clear. Any less than that will require fender mods on most cars anyway, body tolerances being what they are. I'm at 6.1" of backspace with my 18x9's and 275's. My fenders aren't rolled, but the lower front corner is pushed out further than stock with a modified brace, and when I mounted the fenders I pulled them all the way out on the mounts. The tires clear but just. If they came out any at all I'd have to roll the fender or raise the ride height. And the fender roll wouldn't buy even a full 1/4", not on my fenders anyway.
