68 B'Cuda

I have 255/60-15's on the back of mine mounted to 15x8 wheels that have 4.5" of back spacing. I do not have relocated springs and there is about 1/2" of clearance between the tire sidewall and the spring and about 1.5" of clearance to the outer wheel well lip.

A wheel with less back spacing (better centering in the wheel well) would have allowed a 275/60 with no issues.

If you are going to relocate the springs in board of the frame rails then there is a lot of room in the 68 Cuda wheel wells for a much bigger tire.