If it is an air conditioned vehicle, make sure that there is not an accumulation of road dirt, bugs, or other material restricting airflow through the condenser; you would be amazed what a pressure washer will flush out of there! Remember, all of the air must makee it through the condenser core before flowing through the radiator.
Even though you aren't showing any other symptoms yet, performing the chemical test for traces of exhaust gas in the coolant might be helpful, just as a double check. I have seen a couple 440s which had blown head gaskets (years back).
If the engine has an exhaust heat riser/ crossover through the heads to warm the carb base on a cold engine (I dont recall if this car has one and none of my manuals go back that far), make sure that the heat riser valve is fully open when the engine is warm. This may involve dropping the header pipe enough to physically inspect the valve to make sure the butterfly has not come loose on the shaft.
It does sound like you have covered most potential problems already. Perhaps trying another thermostat? If it is a discount parts store thermostat and not an OE one, it may not be opening fully. It might be interesting to try removing the thermostat temporarily to see if the problem goes away.
That is pretty much all that occurs to me at this time, if we think of anything else I'll be sure to let you know.
If I can be of any further assistance, please do not hesitate to ask...