I have never hear of of using cold water during break in. Infact the idea scares me a little. Your system needs pressure to stop the boiling of the water. You would think that the cold water would keep it cooler but once the area around the heads gets heat soaked the water cannot really get that close to it because it is steaming off so fast. I would also be worried about the hose having the needed volume.

All I used to do on the race engines was put a big shop fan in front of the radiator. That worked great. As for the thremostate we used to used a restrictor instead of the stat. We found that over 5500 RPM the coolant was not tranfering the heat like it did at 4000 RPMs. We just made our own as it was just a pc of sheet metal with a hole in it.

 

Also, an unpressurized cooling system is going to generate cold and hot spots throughout the block. That aside, the first time I saw headers glow red, actually yellow, was way back in my high school days, when one of my buddies installed the camshaft out of time in his flathead Ford. Sounded good & ran like crap.

 

Just FYI on a rich engine the header will run cooler. Without air injection there is no air in the header to burn. Lean burns much hotter. Two things will cause a hot header. Increased pressue and lean mixture. Pete example of the Cam timing be off is a perfect example of incresed pressure. The valve is opening while the piston is still in it compression stroke.

Now if we where talking a 80's air injected manifold you would be right. I have seen some pretty hot Y pipes.

 

Don't always trust the timing mark unless you verified it when you built the engine. I had the same issue with mine and found out my timing mark was off by approximately 9 degrees. I bumped my initial up to about 25 degrees on my balancer (14 degrees actual) and the car ran way better and the headers stopped glowing. Does the engine sound or feel lazy with the current timing?