Just a thought.....you mentioned PO....have you been driving the truck with normal temps and this was just a sudden occurance, or is this a new truck to you that has overheated from day one?
If it just started overheating after the thermo change...did you make sure the thermo housing was clean where the thermo seats and is it installed with the correct side up? If you got it clean, so the thermo functions properly then I would suggest putting a Stant pressure tester on the radiator..with a cool radiator...fill rad....crank engine....pump up the tester to the correct system pressure (say 13-15 pounds), and watch for the gauge pressure to either build or drop. If it builds up pressure before the engine gets hot you've got a potential head gasket problem. This buildup will push the water past your pressure cap....engine will overheat. If the pressure drops you've got a leak and it should be evident dripping. If you've noticed antifreeze under the water pump the pressure should eliminate any doubt as to the problem. Hopefully, the hot empty radiator hasn't had cold water added without the engine running....great way to crack a block.
The Stant radiator testers are an invaluable little tool to have in a restoration shop. They take the guess work out of engine diagnosis and can verify that your cooling system/cap is functioning properly. Adding a coolant recovery system for $10-12.00 from most parts suppliers will keep a good cooling high pressure system from belching out small amounts of coolant. All of the guys suggestions are good advice as long as you don't have a leaking head gasket. I hope this helps.
