Most likely the radiator. You can remove it and have it roded and dipped to clean it like new. I'm also assuming that this engine came in the car, I put a built 383 stroker in a 64 Chevelle that came from the factory with a 6 cylinder. I was always running hot until I changed out the radiator with a bigger more flow model. Good luck.
1 good radiator
2 thermostat in upside down
3 needs tune up... advanced ignition/rich running causes extra heat
4 coolant 50/50 coolant/water...look at a product called wetter water at parts store
5 hoses good...soft hose flattens out when hot
6 fan working ok?
7 exhaust system OK? cat plugged or flat/bent pipe
have you checked the oil to see if there is water in it it will look white and be over the full mark on the dip stick it could be a bad head gasket if the radiator is that new i don't think it would be stopped up i agree with the one that said the thermostat could be in bottom up i have bought bad ones from the auto parts store if nothing else you can take the stat out and bring a pot of water to boil and sit the thermostat in the water and watch to see if it opens remove and watch to see if closes back. if no movement replace it its hard to tell over the computer hope you get it fixed.
1 good radiator
2 thermostat in upside down
3 needs tune up... advanced ignition/rich running causes extra heat
4 coolant 50/50 coolant/water...look at a product called wetter water at parts store
5 hoses good...soft hose flattens out when hot
6 fan working ok?
7 exhaust system OK? cat plugged or flat/bent pipe