245/60/15's on 15X8 rims

Getting ready to purchase my new rims and tires for my 68 GTS but not to sure what will work on the rear. I'm looking at 245/60/15 on a 15X7 or 8 inch rim or 255 60/15's on a 15X8 inch rim. Suspension is all stock except air shocks right now but plan on taking them off and putting regular shocks on and adding a leaf spring to raise the back end. On the left side I have 11 3/4 inches between wheel lip and inner fender and 12 inches on the right side.