Hy RogerB1941GMC, I believe the factory used 180 degree F thermostats. The truck probably came with a zero pressure cap when new, so any pressure would help stop the coolant boiling, you could contact the radiator company and ask if they have a suggestion. Years ago the factory started putting 190 degree thermostats in the engines, this made me nervous as the temperature was now ten degrees closer to the boiling point, but it doesn't seem to have created the problems I was fearing. Hope that helps.
