If there is a blown hg pushing combustion gas in to the coolant then there will be significant bubbles in the tank, it could bubble out. However even when that is the case you can run the truck and not have coolant come out of the bottle. Run the cap one turn loose from full tight (360 degrees) and the bottle about half full. If you suspect you have a blown HG you can start the truck from cold with the cap off and look down in the bottle to see how many bubbles you have - post a video.

 

HG was replaced during a complete engine rebuild by a pro shop <7k miles ago, but I did an exhaust gas check on the bottle. Ran a rubber sealed CO2 catalyst tester to the coolant tank for 25 minutes of combined idle and 3500k RPMs and it did not react.

Any other ideas what else could produce increased pressure to cause coolant to come out the overflow hose?

 

Bad rad cap will do it