Ken, After driving cross country in 1999 in a`37 Roadmaster with overheating problems, I finally figured out how to cure it. I tried all the tricks...new 5 row radiator, rebuilt water pump, adjusted the timimg, used water wetter, ran with and without a thermostat, ran with and without antifreeze and use premium gas. Nothing helped. You mentioned the first step....blocking off the bypass valve. I used a 1 15/16" freeze plug with the lip ground off and inserted it in the hose between the water pump and thermostat housing after removing the valve in the housing. The second step was to replace the radiator filler neck with one that would accept a preesurized cap. Then added an overflow tank. The top tank took 7 lb pressure without ballooning. I think a 7 lb cap is the max unless you stiffen up the inside of the top tank. I made 3 more cross country trips after the "fix" without any overheating. Good luck, Bob www.greatrace.com