Did you find out if you have low compression pistons like 8point said? If your not sure, did you ask your engine builder?
Maybe the balancer is off. Stealthbird mentioned timing should be around 34 degrees advanced.
Have you used a coolant tester? Suction the radiator fluid when cool and see what it says. I'm not sure if Water Wetter will effect this measurement. With Water Wetter, more water than antifreeze might be helpful. Make sure your using the appropriate amount of Water Wetter for the number of quarts of fluid. This article may help: http://www.racerpartswholesale.com/downloads/WaterWetter.pdf Water Wetter may help the overheating but I think there is an underlying cause that needs to be addressed.
Maybe a new radiator cap might help. The cap should be 14-17 psi on a 400/428. I don't think the 455 would be much different but I may be wrong.
What was the weather temp the day you tested? If it was hot, it's easy for cars to overheat during idle. Like Vikki said, airflow is needed.
"An ignorant man thinks he knows everything, a wise man knows he doesn't."
