14s will look fine with the 15s, the difference is barely noticable.........it was a very common combination in the '70s. It's getting harder to find 14 inch tires though, not impossible, just harder. Something around 25 inch should do it. Presume you're going hiboy? One other consideration is brake to wheel compatibility, if you don't have the front brakes make sure you get some that will fit inside those wheels.