I'm going through the same decision process on my 67 GTS, stock 8 3/4, stock disc/drum brakes, SS springs. I want to go with steel wheels 15x7, 4" backspace... with largest tires in stock wheel wells/location that allow good drivability(not rubbing). Do you think you could've fit the 255 60 15s without moving the spring in, with 4 inch backspace? When I measured, it looked like it would be tight but good.