OK I installed a 1991 5.0 HO engine into my 1967 mustang which was originally a 6cyl. we swapped out the timing gear cover and put on a clock wise water pump for a V belt for a 1970 mustang. I'm using a cast iron four barrel ford intake from an early model car. I have not replaced my radiator but i have one coming. We have pulled the heads and replaced the gaskets put new rings and bearings and honed the cylinders. we checked and the gaskets have the tabs poking out of the front of the head so we know they installed correctly. The engine runs fine but it keeps pushing the water out of the radiator until the water level gets low then and only then it starts to over heat. We have checked the spark plugs and they are not oily or steamed cleaned. We have replaced the thermostat twice. The engine runs for about 5 mins then the intake and the valve covers are very hot. There is not any water in the oil. please if you have any suggestions I'm at my wits end.