If you're running the original radiator, a high pressure cap will destroy it. It was designed for a 4-lb. cap, I believe. Later models ran a tubular type radiator that will handle much higher pressures than the honeycomb core that was used with the original 6-cylinders.
If you've fitted a modern design radiator, or had yours recored with a tubular type core, the 14 to 16 lb. cap is correct. Otherwise, run a 180 degree thermostat, and plenty of fan, to be sure the temperature stays below 200 degrees at all times, and run a 7-lb. cap, max.
Jerry
