If you don't have a leak not bubbles or coolant in the oil than you are probably wasting your time with a rebuild. Need to check the thermostat and water pump. Pull them out and inspect. Make sure your belt is not slipping. Pull the belt and see how tight the water pump is, that will be a good indication of the bearing in the pump. How do you know the radiator is good? Pull the hoses and run water thru it to see how it flows, it could just be clogged. Do you see coolant flowing when you pull the radiator cap? Pull it when cold not hot and let it warm up. Should see coolant movement. You need to do a lot more investigating before you open up the engine. Odds are if you rebuild it, it won't fix it.