Exactly what is the engine and what is it in? If it has the original GM distributer advance the whole body will twist when you hit the throttle. If it is something later only the point plate moves. If it is HEI or some other electronic crap you need some diagnostic equipment.
Yes, I meant that the heater core gave added cooling capacity, extra to the regular cooling .
Are you just going with the gauge reading or is it actually boiling over. Is it some sort of stock gauge? Old GMs had a 168 degree thermostat. This was moved up to accommodate emissions standards to 190-195. Old gauges will show overheating in this range. Water boils at 212 degrees F with no pressure. The more pressure the higher the boiling point until the whole thing blows up. If it does not look like a steam engine waiting to load passengers at the station get it out on the road and see what happens.
The further the fan is from the radiator without a shroud the less air is pulled through the radiator core especially sitting still at idle. The tighter the & deeper core the worse it is. Extra space allows the fan to pull air from around the radiator not helping cooling and just adding fan drag to what work the engine is doing.
