67 Ford Mustang Gt 390 Overheating Issue

Have you thought of installing an aftermarket gauge to see what the temp is actually getting up to? Is it forcing any coolant out the overflow? Did you go with the same cam or a beefier one? What temp thermostat are you running? I would look at fuel mixture. If you're running a somewhat lean mixture your engine will run hotter and especially at higher rpms. If you went with a beefier cam and you have a stock radiator maybe go with a bigger aluminum radiator with a high flow electric fan. I would personally install an aftermarket temp gauge and see what its actually operating at and if its not actually overheating then I wouldn't worry.

I did not see it there's overflow or not but another common cause can be the spring pressure in the radiator cap fades over time. Of course that would result in blow off or over flow.

I suspect the hotter cam is just that...hotter. I also second the advice of finding out what the temp actually is; even is the sender and gauge are not malfunctioning they were never really accurate.

I'd recommend.....

1. Drain and flush cooling system with plain water.

2. Remove thermostat and reinstall housing.

3. Fill with 8 oz. of "Shout" laundry stain remover and water and run for 30 minutes after it warms up.

4. Drain and flush with plain water.

5. Refill with Mercedes-Benz citrus-based flush according to directions.

6. Flush with plain water. If you get a lot of crud you may want to repeat step #5.

7. Refill with premixed coolant or 50/50 antifreeze and distilled water.

I usually refill with the front wheels on ramps so the water neck on the radiator is the highest point in the system to get all the air out.

I really doubt that a new water pump will fix it. It sounds to me like the cooling system was marginal to begin with and adding the cam (more power = more heat) was the last straw. That, or your lower radiator hose is collapsing at elevated RPM. It should have a wire coil inside to prevent that, although I would expect this to have been a problem before as well.

As tos said, checking the mixture is a good idea too. It's a good idea anyway, but when I had a '65 Mustang with borderline inadequate cooling, it was very sensitive to the mixture.

I'd skip the electric fan, though. The alternator should be upgraded in order to run a fan powerful enough to be worth having. I would opt for a high-flow rigid fan with a thermostatic clutch instead. Simpler, easier, same results, and a more vintage look to boot. Also use a shroud if you don't already have one. They greatly improve airflow through the radiator.

If you didn't see a lot of rust or barnacles in the block when the water pump was off, then I think doing a whole-system flush will be a lot of effort for little gain.