A few questions, How wide is the steel wheel? What off set? Front or rear or both? Stock hight or lowered? Higher than stock ride hight?
My 63 Valiant I am running steelies from a 84 Dodge Diplomat that are 15x7 with a 4 1/4 backspace. On the rear 205-65-15 and can can go larger(235 60 15 no problem). On the front 185-65-15.I am running the 73-76 a body factory disc brakes with 2'' drop spindles.
The wheel openings are smaller on the 63 fronts and had some rubbing in turns.Your 66 should clear a larger tire. Hope this helps.