There are 2 kinds of radiator caps. Ones that allow coolant to be pulled back into the system, and ones that don't.
Like Paul says, a higher cap pressure would slow down the overflowing. A word of caution, adding more pressure to these older radiators and heater cores may result in a trip to the radiator shop because of leaks. At any rate, 7 lbs. would be the minimum that I would run, more if it can handle it.
