It could be that it's a new motor and new motors make a lot of heat because of increased friction by pistons on freshly honed cylinder walls. Retarded ignition timing causes overheating. Lean fuel/air mixture, maybe? And, 250lbs compression is a lot. Most gas engines run 150-185psi. It becomes a diesel when you get above 400psi. Do you have a shroud on the radiator? Some of the old 'vettes didn't have one and that caused overheating.