tolerance issue?

Ok I have been fitting tires on a pair of 15x7 cop wheels on my 66 barracuda. I have replaced the stock rear with an 8 1/4 out of a 74 dart. I put a 225/60/15 tire on the driver side rear to try out. I have less than 1/2 an inch clearance between the quarter panel and tire. it doEs not rub though. On the other side when I put the same size tire and I have at least an inch clearance. Is there a good reason for this? Could the tolerance from side to side be off that much? How can I get more room on the driver side?