A 7# cap will raise the boiling point 21 degrees, to 233. At 215, you're getting dangerously close to a boil. It looks like you need a little more cooling capacity, or at least a recore to a higher-pressure system. With the right size radiator, the coolant temperature should stay at, or slightly above the thermostat opening temp. I'd suggest going to a new aluminum radiator, or get your present unit reworked for a high-pressure tubular core capable of handling a 15# cap.
Jerry
