What radiator
What water pump
Did you over rev?
Did you torque the heads properly
Do you need sealer on some head bolts like a small block
Is there coolant in the
oil
Is there
oil in the coolant
Did you bleed the cooling system properly
How did you break in the engine initially
You can pressurize the system like mentioned above and also put a pressure gauge on the filler and rev the engine and see if you build up pressure. You can also do a dye test for emission gasses in the cooling system.
You may want to read this
http://www.troubleshooters.com/toverheat.htm
Roscoe