Thermostat or No Thermostat?

I know this is an old thread, but I think the problem is due to the .40 over bore. With a .40 over bore, your new displacement is 435 cubic inches. A 435 cubic inch engine will make a lot more heat than a 359 cubic inch. That's a heckuva bore! :D

I jest of course, I realize you meant the engine was bored 0.040". :D