DocDirk Doc you shouldn't need anything more than a 15# cap. What thermostat are you running in the motor? Run a 180 temp would be about the best for your area. Do you have a shroud on the coolant fan? You want to pull as much air through the rad to control and lower the coolant temp. Water pump, who's do you have? Here's the best one, are you running UNDER DRIVE PULLEYS???
from March?? They don't spin the water pump over fast enough to cool the motor and control heat. If the car sat for a couple of months you might want to replace the thermostat first. Drill a couple of 1/8" holes in the edge of the thermostat to help remove air from the coolant system. Make sure that is not alot of debrie in the fins on the radiator. One more check is to make sure that the rad has no cool spots. This could mean that the tubes are not flowing. Anything more than 16# cap you are looking at possible loosing on a hose to this much pressure. How low has the motor been built and running. Do you hear and bubbling or gruging sound coming from the motor when hot? Before you start playing with anything, get a infrared temp gun and check to see what the coolant temps are around the motor and rad. It is VERY POSSIBLE that your coolant gauage could be 10-20 degrees off and there is no problem. Let's do the basics first before we go hunting for a nightmare. This has happened before to other guys here. Rick L. Ps you have PM.