Rino, in your situation, I'd stay with the higher-pressure cap. The 7-lb. maximum serves to protect the old style honeycomb core. If it's got a heavy-duty tubular core, the higher pressure adds some insurance against boil-over when the temperature spikes. If you're getting a spike, then a quick cool-down, it's possible the thermostat is sticking closed occasionally. Even new ones can do that.
Every pound of pressure adds 3 degrees to the boiling point, so a 15-lb. cap allows a boil-over temperature of about 257 degrees at sea level, lower as you go up in altitude. Unless you're in traffic with the air conditioner on, or pulling a heavy load, etc., the temperature should be pretty stable at or near the thermostat opening temp, assuming you've got enough radiator capacity to cool the engine properly. One of the biggest mistakes some hotrodders make is to add a big engine without an appropriate increase in cooling capacity.
Jerry
