’67 Camaro – Overheating at highway speeds

Hi all. The 350 in my ’67 Camaro seems to want to slowly overheat at constant speeds over 70mph. Car stays nice and cool at idle or city driving (185 degrees), but once over 70, it has gotten as high as 212 degrees. Might have gone higher but I pulled off.

So, I’ve got a stock setup with factory fan, clutch and shroud. Fan clutch and water pump are new, but the car did this prior to replacing those, whose replacement was unrelated. 180 t-stat is new, but doesn’t mean it can’t be faulty, but I know its opening, as the car stays cool at idle and cruising speeds. No white smoke out of the exhaust, and oil is clear and light amber in color, so I don’t ‘think’ its a blown head gasket. Car also has a new radiator cap.

Over the past couple of weekends, I’ve checked coolant levels and the radiator has been full every time, except for today. It was half full and wondering if it was ejected when the car was hot today, although seems to be unlikely as I didn’t see it, and the car didn’t go past 210. I would think that a water/coolant temp would have to approach 240 degrees before this would happen? There is no overflow receptacle, just a tube that sends overflow to the ground.

I haven’t tested for lower hose collapse, although the hose does seem a little more flexible than I thought it should be, but I compared to a new Gates hose and it didn’t seem to be that different.

I’m not sure how old the current radiator is, but it looks somewhat stock and the coils inside are also rusted. I did a pretty extensive reverse flush a few months ago, and I’m already getting brown residue coming off of the cores. So it is possible that I’ve got some kind of blockage/sludge in there that is only detrimental during higher speeds and water flow.

My plan is to replace the radiator, hoses and t-stat this weekend. Anyone have any suggestions otherwise?

Thanks!