I currently have a set of 15x7 Salt Flat Specials on my 64 Safari with 25560R15s on the rear. They will definitely rub if you don't trim about 1/4" out of the inner wheel lip. The 15x7 with the American Racing standard backspacing of 3.75" is just fine but you can't go with a very wide tire. The sidewall bulge will drag. Hope this is a little helpful.
John
www.fullsizepontiacs.com