You probably should have started your own thread on this which would have created more attention for you. It's going to be a long guessing game which also takes away from this one I started.
Because the radiator is cooling fine, it sure sounds like a water circulation problem inside the engine. Maybe it's a longshot, but being a new rebuild, I wonder if incorrect head gaskets or intake gaskets were used and the coolant passages are partiallly blocked off. Good luck.