The 5" BS will help but a lot of it comes down to the section width of the tire. I looked at section widths as much as I did the tire size. On my 67 I'm have 15 x 8 on 4.5"BS steel wheel with M/T ET street 255/60 that fits in there nicely. A 275/60 will work with a 4.5 BS on a 7" wheel with a smaller section width but it will still be tight on the quarter panel side. These cars are very space limited as opposed to earlier and later B bodies.
when you say tight on the quarter panel side, is that at the lip of the wheel opening or is it where the inner wheel housing curves in towards the wheel a few inches above the wheel opening?
