Interesting subject thermostats Heinz. Not all of a particular advertised temperature work the same. The opening range is equally important. I was given an "80°C" thermostat to cure overheating on a vintage marine diesel engine, replacing the previous 72°C one. It worked because the high temperature one actually started opening at 65°C. I suspect a new thermostat alone would cure your issue Heinz although admit a lot of thought, sourcing and experimentation would be needed. This would allow full use of your larger capacity radiator, possibly required in extreme circumstances.
Note after 2006 MMC fitted the same radiator to all Classics so the cooling system didn't have the benefit of custom tuning of system that Ford would have made to each of its cars.
