He has a 1968. And from pics I think I see it is likely original as it appears to still equipped with an 8 bolt waterpump setup. Those pumps don't have the sleeves you refer to like the 69 and up 11 bolt pumps have.
I agree with you 100% that his fan is very visibly too small and poorly fit to the shroud.
Boomstick, your fan needs to be replaced. I also can't see if your filler panels up top are missing above the radiator support. Or below. I know earlier I said the fillers don't matter in your case...but that's when I thought you had switched to an electric fan setup and was later informed otherwise. With your stock-type fan setup the fillers DO matter. But only at highway speeds. At idle, it's all fan...and your fan is poor. But because that fan is very visibly incorrect providing poor idle air flow, plus you've done hop-up mods to the motor...I can fairly easily see this motor doesn't stand much of a chance to properly cool at idle.
But it shouldn't overheat THAT quickly. So again, I believe airflow may be only ONE of a few problems you likely have.
Which brings me to ask you to oplease share your results of you pulling the water pump checking diverter-plate-to-impeller clearance with a feeler gauge.
AND...now that we have more history...MOST IMPORTANTLY: What was the result of the pressure test very smartly suggested by go2fast? A proper pressure test, in conjunction with a cylinder leakdown test, is really the only way to ensure your heads were installed properly. Your earlier different topic that showed ONE badly cooked rocker arm is another big clue. You assumed at that time it had an oil delivery problem to the one rocker, but I now believe you may have assumed wrong. When one localized spot on the engine gets that hot and cooks the oil, it's an indicator of a very localized lean/hot spot...which is a big clue of a head leak at...dare I say the #4 cylinder? (because this is the rocker that cooked). AND...when it got that hot, could it have warped a mating surface to worsen the leak? Yes. As much as i know you hate to hear this, it needs to be said: I now believe something has likely gone wrong in the assembly of this engine, and it has been overheated multiple times in testing which likely made the problem worse, and this likelihood of an internal engine problem needs to be thoroughly looked into, before anything else.
If I were you, no matter the results of the leakdown test, I'd be pulling the head on the side with the #4 cylinder and looking inside to see what's going on, and then checking mating surfaces for warpage before reassembly.
At the very least, you can easily pull rocker cover again and look at that same arm, and see if it's cooked itself again...
