Hi Listerational, you definitely have a problem there, water boils at 212 fahrenheit at sea level, you are way above that. The only way to stop it boiling at that temperature is to use a pressurised cooling system. Lister/Listeroid cylinder head gaskets do not like pressurised cooling systems. A water pump probably won`t do anything as it is clearly thermosyphoning OK. Four choices: 1. a larger radiator, 2. swap the radiator for a vented hopper, 3. fit an electric fan to the radiator, 4. fit a pulley driven fan.
You should be able to find a suitable fan at a local scrapyard, if you are going for an electric fan try to find a pipe thermostat to turn it on and off, otherwise it will take a long time to warm up.
Bob