Not a Chevelle but it's my brothers all new restored 1966 Mustang convertible noted for cooling it’s problems.

New stock radiator, 289 eng. bored 30, over mild cam, Edelbrock heads, cast iron headers. Added an electric fan and shroud which helped considerably. Before had a shroud and new fan clutch, radiator would boil. Test run today on freeway had to slow down from 70 to 55-60 other wise temp kept rising. Seems like over heating could be related to speed of engine. Are the piston walls to thin due to the 30 over bore? . I didn't know until later the thermostat was out.

Thanks in Advance :confused:

Put a t-stat in
Check timing---- and vacuum advance-----with a cam change, stock timing values are gone.

Also might want to make sure it is not running too lean

Been a loooong time since I messed with a Mustang, cant even recall what the timing specs were (factory solid lifter 289). I called it my 'Little Red Death Sled'

Also, I've heard that some electric fans will actually restrict air flow at highway speed, might need a few 'pass thru' holes in the shroud.

Bryan is spot on with the tstat, cooling systems are designed to have them, if they are out the coolant will actually flow to fast through the radiator and not have time to transfer the heat to the cooling fins in the radiator.

PUt a 180 degree stat in it and the problem should be solved.

If your timing is OK (check it, don't assume it's OK) - you probably need a better radiator. Note it's a closed system, the Tstat doesn't matter, it keeps heat in the engine so it doesn't stay too cool but won't help/hurt it if overheating.

Other possibilities:
Bad water pump (impeller blades corrode)
collapsing radiator hose

But really, sounds like you're not getting the heat transfer through the radiator.

I am just writing to reiterate the tstat does mater as I described in my erlier post. I am an ASE master certified auto tech with 15 years on the job. Im not trying to be snobby Im just trying to let you have some of the knowledge I acuired from experience and over 80hours of classes per year. Good Luck

Right or wrong, it's certainly the easiest and cheapest thing to test - put the 180° stat in and see what happens!

Bryan is spot on with the tstat, cooling systems are designed to have them, if they are out the coolant will actually flow to[sic] fast through the radiator and not have time to transfer the heat to the cooling fins in the radiator.

The water can't flow too fast through the radiator under any normal operation, it transfers heat 100% of the time it's in there, and it's the same water over and over. The faster it flows, the more heat is transferred and the more efficient the radiator is. One thing that a thermostat adds is turbulence, which can effect the cooling ability markedly.