Of course a 15x7 will fit with the right backspacing. Not arguing that at all. What tire will fit is a different story.
As far as what you know about 255's, you know they fit
your car. And a few others no doubt, there's a few in this thread. But that doesn't mean they'll fit every car, because they're not all the same. Here's some folks that have encountered cars that either couldn't fit 255's, or had to trim the quarter lip to do it. On 15x7's too.
These are all from this one thread
1969 dart wheel and tire size?
1/8" clearance to the quarter won't work, it WILL rub eventually. One finger width isn't enough either, unless someone's got a 1/2" thick finger. And then there's the "fits on one side, rubs on the other" issue. These cars are not all the same, not even from one side to the other. Axle housings are sometimes offset by a 1/4", wheel wells/quarters can be off by as much as a 1/2", even side to side.
Like I said earlier, that's not enough. 5" to the quarter, 6" to the springs. A 15x7" rim is 8" wide outside lip to outside lip. A 255/60/15 has a section width of 10.2", a 245 is 9.8". So that means you have 1.1" of tire overhang with a 255, and .9" with a 245.
4.25" of backspace means 3.75" of front space. So, 3.75 +1.1" is 4.85". That means you'd have .15" of clearance to the quarter, and that's not nearly enough. It would also mean you have .65" to the springs, which is plenty. Even if you assume the 255's section is squeezed down on a 15x7" and call it 1" of overhang you still don't have enough room. Even with a 245 you'd probably not get away with the 4.25" of backspace, it might clear most of the time but if you did any kind of serious driving or loaded up the trunk or back seat I bet it would rub.
What you want, if you want to run 255's, is a 15x7" with 4.5" of backspace. That's the ticket on your car with BBP axles and brakes. Even for a 245 that would be what you really wanted for backspace.