Carefully clean the old gasket material off the thermostat opening. Use a new gasket when reinstalling the thermostat housing. Don't overtighten the bolts. Torque them to the correct torque setting. I used 25 ft/lbs.
The car runs much cooler now. I have had no problems with engine performance or gas mileage. I recently took a 300 mile trip and averaged 23mpg overall. This is a good "starter" project if you have never dug into your engine. You should be able to do this in a couple of hours. GOOD LUCK!