Dana67Dart said

My 67 273 would boil over especially after shut down. until I started refilling the coolant with the car on a slant (nose up). My driveway is sloped and I put it on ramps. that way the rad inlet is higher than the rest of the system.

the gergalling you hear under the dash could be trapped air in the heater core. Mine did it for a few weeks from the last refil but last weekend I changed out the the T stat gasket and din the noes up refil.

Also if the fluid is traveling too fast through the engine / radiator it can not pickup or expel as much heat. I run a 185 thermostat and a 16 lb cap, factory pulleys. There is no fan shroud and the stock fan sits ~3 inches from the radiator. It will idle all day that way and when I run it on the hwy the temp goes from mid gauge to a few ticks over the low side of normal.

IMHO the issue you are having has to do with engine RPM higher = not cooling as well

I would look at the spark curve as mentioned before and also see if you can see the impeller through the lower radiator hose to be sure it is intact. measure your crank pulley and your water pump pulley and ask here if they are OE.

Drain the fluid out and refill either with the front end up or you can fill with a funnel through the radiator hose till the upper fitting on the manifold shows fluid then reattach the hose and finish filling in the radiator. also make sure the heater hoses are on correctly (in 67 273 the drivers side heater hose goes to the water pump by the bypass hose.)

If it was me I would put the stock thermostat back in what ever temp rating it was OE

good luck

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