Classic/Antique Car Repair: 1965 289 mustang fastback overheating, radiator replacement, head gasket

Question
Hi, the engine gets very hot at highway speed. At idle and cruising at 40 mph
the temp is normal. The water pump is new, lower hose has spring in it and
heads are rebuilt. Also checked that the head gasket was installed correctly
(front label faces to front of engine. Any idea? I also noticed that the head
gasket covers one of the water jacket ports in front of engine and the other
port has only 1/8" hole in the gasket. The gaskets are correct for engine but
why block one port and restrict another? Any suggestions. Thank you.

Answer
It sounds very much like a partially plugged radiator. There is enough flow to remove the heat from lower fuel use running like low speeds and idle. But when you ask the engine for more power by burning more fuel then the restriction slows the flow of coolant enough so that heat is not removed from the engine. Most of the time this calls for a radiator replacement as the stuff gets so hard and crystallized in the radiator that flushing will not touch it.
Brad