coolant expansion tank

I would disconnect the hose to the expansion tank and plug it.

I did. It is not holding pressure. I installed a 160° thermostat, and the temp will not go below 200°. I am assuming it is the expansion tank.

If the tank is disconnected then how could it hold pressure or vacuum? A 160 degree thermostat will not make your truck run cool in the summer and it could cause problems if it did. 200 degrees is fairly normal and good for a Chevy. Use Water Wetter which can reduce operating temps by 8 degrees.

The holes were blocked, and Owas using a coolant system vacuum pump. I have had the computer reprogrammed, and the fans are set to come on to run at 160.

If you cannot pull a vacuum then there is a leak somewhere. Did they program the ECU to stop fuel enrichment at a lower temperature? Good luck in your quest to have a Cool Chevy.