Maxwedge is telling you the truth!!! This is a simple and over looked problem throwing parts at it will not work. A vapor tester over the rad opening could have proven it was not a bad head or gasket. I suggest checking the timing mark on the harmonic balancer,is it true. Are you running the carb way to lean (check the spark plugs color, use the AC's or CHAMPION's).
Has this engine been rebuilt or long time storage? Have you tried a coolant flushing?