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.