cloged radiator Ya know.. going back to a fluids class i had about 2 years ago.. A clogged radiator would make sence. at idle the pump would pump fine while drawing a little vacuum. At highspeed the pump could cavitate (causing air at the impleller) if the vaccuum was too great at the input side of the pump. check the hose and see if its trying to collapse itself at higher RPM you may even be able to feel pulsating in the intake hose as the pressure changes from the cavitation. just my 2 cents Nate
Do check the coolant flow issues. That said, Nailhead hit another good place to look, ignition, mainly the distributor. Advance curves, vacuum advance issues can do just this kind of heating problem described.
My bet is on a clogged radiator... And if I remember right, overflow tanks weren't std equipment until about 1970...My 66 Skylark(no a/c) w/300 didn't have it...My grandpa's 69 Cutlass w/350 and a/c doesn't have it either... BTW, are your drum brakes dragging???
overheating i would pull the rad out of the car & reverse flush it & maybe even pull the water pump & flush the block . i did these functions on my gs 350 & u should have seen the crap that blew out of both the block & rad . unbelievable !!
