Big Block high speed over heat
The radiator you are using is this the original radiator. Did the Camaro originally come with the 396 Cid. Is it a four core radiator? Do you have the fan shroud for a 1969 big block radiator? There are two one is for 23" radiators and the other is for 20 3/4 radiators. Can you get a Infrared Thermometer Gun to check the block temp, temp head, radiator, etc. You can see hot spots in the block and heads. Need some more info such as last time engine was rebuilt, clutch fan. Cast iron or aluminum heads. Have you done a compression test or leak down test for checking the head gaskets.