Jerry is spot on, with thin cylinder walls the heat gets into the coolant faster than the radiator can exchange it. A very free flowing exhaust will help with large over bores since about 85% of an engines waste heat leaves via the exhaust system but there is a point of no return between heat generated and heat expelled. We raced Hudsons with no heat problems but the flathead Ford guys had some exotic (and some ingenius) ways to get rid of heat. Cowling (flaring, baffling) also needs to be where all the incoming air is directed through the radiator and the heated air has a no restricted path to exit the engine compartment. Splash pans along each side of the engine may have to be removed and sometimes the inner fender will have to be louvered. It's not called a cooling SYSTEM for no reason.
