You should not run it with no thermostat, the thermostat act as a flow regulator. Without one, the coolant flow is too fast, and will not be able to exchange the heat out to the radiator effectively. If you don't use a t-stat, you can put a fender washer in the neck to slow the coolant flow down to an acceptible rate. That's an old Flathead trick. Personally, I think that a 180 t-stat will be fine, used witha 50/50 antifreez/coolant mix. Something most people forget is that "antifreeze" is also a coolant. It actually raises the boiling point of the water in addition to lowering the freeze point. It also has anti-corrosion additives in it. I'm running a 50/50 glycol (green) mix with a 160 t-stat and a bottle of Red-Line Water Wetter, the '55 runs around 170, a little higher in traffic.
