If you are running conventional water-based coolant, a 7-pound cap is not correct. It will reduce stress on the radiator and other potential leak points, but it will also fail to prevent localised boiling inside the engine which leads to ping. Better to have an intact cooling system and use the correct pressure cap.
Me, I run
waterless coolant and so I run a 7-pound cap, but only because a 0-pound (non-pressure) cap isn't readily available.
Stock pressure rating on the cap is 13 or 14 pounds without A/C, 16 pounds with A/C. See
here for info on different types of caps.
You'll want to measure with an actual thermometer to see if your high temperature gauge reading means the engine is running hotter than it should, or that the gauge is not accurate. You'll also want to do normal maintenance -- thermostat (180°, preferably a Stant Superstat #45358, take a good look at that radiator, make sure the hoses are in good shape, and keep in mind two frequent causes of heat-related issues on slant-6 engines: rust-through of the coolant blockoff on the original shim steel type head gasket (which allows coolant to short-circuit its intended flow path, leaving some cylinders inadequately cooled) and "mud" buildup in the block, especially at the rear (which insulates the cylinders and prevents effective cooling). But if your gauge is reading hot and the radiator is suspect, I'd start looking there.
