Stuck thermostat bolts

Have you tried spraying it down with some diesel fuel? Let it sit overnight.

If you repeatedly apply torque and back off or go back and forth with the bolt it creates stress on the binding threads and can eventually come lose , you have to go easy . One other option is the hot wrench , they sell an O2 - MAPP gas combo torch set that can get things red hot . But in your case I'm thinking there's not much room and the flame will snuff out . Many times just a little heat does the trick ..