I've got a '66 with a 289. Put in a larger cam & hi-flow water pump + an aluminum dual-plane 4bbl intake (yes, the manifold gaskets are installed correctly) under a Holly 600 carb. Otherwise block is stock bore with heads off a '74 302, also have an electric fan that cycles on at 180 degrees pulling air through a 3 row aluminum radiator and auxillary transmission cooler for and aod. Not that it helps operating temp., but dropped to a 160 degree thermostat. Operating temperature stays around 180/190 untill engine is under load then begins to climb to 210 then 220 and boils over. Takes about 10-15 min. for temp to come back to 180/190 when engine is not under load. Hi-flow water pump is new and is operating well-maybe too well? Could the coolant flow rate be too fast? I've heard that if it's too fast it doesn't draw enough heat out of the block as it passes through the water jacket. Also, I'm using a stock radiator cap on the aluminum race radiator-is that sufficient? Any theories fellas?