How does the inside of your radiator look when looking through the cap-opening (w/ coolant drained a bit below the toplevel)?

Are the channel-entries clean and open?

A citric acid treatment cured overheating in a 496ci setup with stock 3-row radiator for me.
The car still has a 1:1 crank:waterpump ratio, but it also has its stock 'narrow' steel shroud with a metal flex-fan partly inside.

Engine has 10.5:1 CR, XR274-solid roller, Edelbrock heads.

Before it had rust and scale-particles blocking the radiator channels and everything had a brownish color.

After the first full day-soak in citric acid, the radiator metal already looked like new, aside from a few deposites near the channel-openings.
After that I let it soak a few days more while occassionally tilting the radiator to agitate the acid inside.