Rear Tire Sizes

I have P255 60 /15s on my 70 Duster, mounted on Vintique 8" wide rallyes. There is about 3/8ths inch clearance to the spring. They only make contact when I jack up one side of the car. Probably would rub going around corners at 100 mph !! Outside - no problems. I use thin spacers for the front wheels so the car wouldn't look wierd from the back.