It's often hard to diagnose symptoms via the internet, but here's 2 questions/suggestions, won't cost you anything but time...

are you using a cast iron water pump?..if so turn the thermostat upside down in the waterneck, flow is reverse...

another suggestion, start the vehicle, put the radiator cap on, but turn it back to the point that it's just engaging the tangs where you have to push it down to release, but still engaged, but loose, run the engine, if it doesn't overheat,...I'd start looking at head gaskets...