I have a 216 motor in my 49 p/u and think it's original. Now that I got it on the road, I noticed that the engine starts knocking loudly especailly after it gets hot. It will overheat a little and spit out some coolant and I have a pin hole leak at the top of the radiator at the filler neck. I'm really concerned about the oil pressure. When cool, it will be just above 15 psi. I have a 30 psi gauge. When hot, the pressure will fluctuate from there and then drop out almost down to zero especially when I'm stopped at a light. The temp. generally stays about 180 but will go up when the oil pressure drops out. The engine does not smoke or use oil and no antifreeze is in the oil. This knocking sound is really loud and I worried about losing the engine. How bad is it???