1974 Chevy Camaro Car overheats

1974 Chevy Camaro Manual

I have a 1974 chevy camaro. It overheats about 10 degrees per like 2 minutes. We have tried everything. We changed the waterpump, changed the thermostat, checked the radiator and it was fine, we checked head gaskets and cylinders and its still fine. The fan works good and so does everything else. There is no blockage in the radiator or anywhere else. We are comepletely confused. What could be wrong? The car has never run that hot and i've had it for maybe 5 years. I've raced it and it still has not run that hot. About the end of last summer it started running hot, I didn't think much of it, but then back in novemember I got it painted, and when I got it back in april I noticed how much faster it started running hot. I've been racing for 30 years with my brothers and even my brother is confused. We have no idea what it could be.