That size should work. Some people cut the opening larger, but probably no need since the inflowing air will find its way to all of the radiator area anyway. I expect you have the 71+ alum w.p. like me, which is 1" thicker than the 65 cast-iron. If you won't have a mechanical fan, it should fit. My nylon fan sits pretty close to the radiator core. I will put a pusher electric fan on the front to assist (Mercedes). Nobody can answer exactly what would work for you. If your engine runs lean or inefficient, you need more cooling. If you plan to climb out of the L.A. Basin at 70 mph on a 110 F day, you could easily join the others with their hoods up every 1/4 mile. Not all new factory designs can handle that.
