First and foremost - get a real gauge on it. I also thought my son's 400 in his '73 RR was running hot. The factory gauge would get up to the line just before H and stay there all day long...but the radiator never puked coolant. I did two things -- put an Autometer Sport Comp mechanical gauge in and bought a hand-held infra red thermometer -- both told the same story. The temp wasn't getting above 180*. Don't rely on your factory gauge...