Actually, even though the 15x7 4.25" wheels are a +6 offset and the 15x8's 4.5" backspace wheels are a 0 offset, the difference in the tire width makes up the difference in offset. The outer edge of the 245's on the 15x8's the OP is talking about should be almost in the same place the 255's on your 15x7's. But, that's just on the math, which doesn't really account for how the sidewalls are squeezed on that narrower rim. That's just going on the section width specs for the tires.
It's going to be REALLY tight to the quarter. On my '71 Dart with an 8 3/4 and BBP axles I have 6" from the wheel mount to the springs (stock location) and 5" from the wheel mount to the uncut quarter lip. Zero offset makes it easy, the section width on a 245/60/15 is 9.6" wide according to the specs and that will stick out 4.8" from the wheel mount. On my car they'd rub, you need a minimum of about a 1/2" of clearance to allow for body roll and suspension movement. On a SBP axle/brake set up you should have an extra 5/16" on each side compared to a BBP axle/brake set up because of the narrower track on the SBP gear, which gives you almost exactly a 1/2" of clearance. But, if the axle isn't perfectly centered, and they usually aren't, they may rub on the side that's tighter. You'd have been better off with a 15x8" with a 5" backspace, if it was a little tight on the springs you could have used a small spacer. If it's tight on the quarter lips all you can do is cut or roll the quarter lips.