[Michael C Henry]

When I was 18 years old I had a Hot 406 in a 55 Ford . I ran no thermostat and a F 100 truck 4 row radiator . No real fan shroud but a wide raditaor . Engine idle was 1500 RPM, really wild cam probably carb problems too. My temps were 200+ in Iowa summer heat.
I'm also thinking you have other problems other than the thermostat. What your seeing is the temperature that is the result of the process of how much heat your creating and what can be lost to the atmosphere. If outside temps go up the resulting engine temp would also be higher, I'll bet. First , are you getting enough fresh air across the radiator fins? You have to get fresh cooler air into the radiator fins and you have to get the used heated air out from behind the radiator and out from under the hood. Does a spray of cool garden hose water into the radiator effect the temp? The thermostats setting is just where it starts. I'm sure you've checked somethings but there are so many points that could cause this. A plugged radiator, thermostat bypass ( that little hose between the intake manifold and coolant pump) plugged, compression leak into fluid system, is the coolant pump actually moving fluid agressivelly. What is the history of this combination? If it worked before ,what have you changed since. If you just had rebuilt the engine I'd wonder if the head gaskets were turned so the large coolant passage at one end was at the rear as it is supposed to be. If one or both head gaskets were positioned so the coolant opening was to the front. This condition would cause the coolant flow to be a short loop out of the pump into the engine block , upward into the front of the head and to the thermostat. Coolant at the rear of the engine would be somewhat stagnet, Letting temps get out of hand ( above boiling) causing gasses which do not conduct heat as well as fluid. It would mix with the circulating coolant and average out.
Thing to check ,start the engine, as system is starting to heat and system is not up to temp and under pressuer yet Squeeze the hoses, is the large upper hose out of thermostat to radiator somewhat stiff and pressureized and radiator return hose ,at the bottom of the radiator going to the coolant pump, softer and collaspsable? That would indicate a plugged radiator. Where is your coolant fill point should be the highest point in the system? If it is between the thermostat and radiator and as ususal when just starting a cold engine can you remove the pressure cap without coolant belching out?
In a free flowing coolant system coolant is pulled from the bottom of the radiator by the coolant puimp and forced into the engine block . It should flow out of the thermostat and fall through the radiator. The pressurized cap comes into play to increase the boilnig temperature point.