Centerline auto drag 15x8.5 on 69 Dart

I have be running 15X7 inch steelies in rear for awhile now .... but I swear my tires are 255-60-15???? I guess this is no point in going to an 8 inch wheel at this time .. can't fit any bigger tire in tub .... So I guess only alternative would be to move springs in ..
Yep, the word on the street is a 255 60r15 will work on most 69's with a 7" wheel but not with an 8". The wider wheel widens the tire. I have a housing I narrowed 1" and this combined with the offset spring location should allow the 8.5" centerline and 275 60r15.