I spent 'way too much money and time with my overheating problem on my BBC. Even went to the point of taking the water pump apart to ensure the impeller blades weren't spinning on the shaft. Never found the culprit. The very same block had run fine for many years at .060 over and the heat started when I cleaned it up with another .010 over bore. Still can't imagine that taking a measley .050 of material off the cylinder walls made the difference but...I finally bit the bullet and ran Evans waterless coolant. Still ran hot, but didn't worry about it since there was never any pressure created ( ran a 7# cap just to keep the bugs out) and the good full
synthetic oil I ran was good for the 250* I'd hit on the strip. Costly stuff, but worth it for my peace of mind.