1987 iroc z 5.7 car 20k original miles .. pulled motor cam lifters push rods , seals, springs , valve job timing Chain etc . All new gaskets .. removed the smog pump and emissions .. Car starts runs sounds great .. valve seals needed changed and a lifter had a small tick .. Car never over heated before .. it builds pressure and over heats fast .. changed thermostat twice .. changed water pump twice .. Radiator was checked by a radiator shop said it’s perfect .. new radiator cap..Any advice is greatly appreciated .. I’m out of ideas ..

 

Heater core clogged?

 

What do you mean "boiling in overflow tank"? As the engine is warming up do you see a bubble every few seconds? Or do you mean after it warms up you see bubbles? Either way you have air in the coolant system that needs to be burped out.

Do you have the correct rotation water pump?

 

BBCSwap - U are correct.