Waggy,
I will amend my thinking based on some of the above posts. Changing thermostats will have absolutely no bearing on the temperature your engine runs. Engine operating temperature is a factor designed in by the engineers in Detroit and has nothing to do with your cooling system. You will have to buy a new engine that has been designed to run at a lower temperature, then the cooling system you now have will be able handle it.
The key point here is that thermostat rating and the operating temperature of the engine have nothing at all to do with each other. I quote
"normal operating temp will not be related to stat temp". 
EDIT. Yes, this post is pure sarcasm. :rolleyes:
Stuart
