I used a Mr Gasket copper seal.

Also helps to take the bracket that held the existing header on underneath the car. It has two bolts to it and goes up by the oil filter'ish (L shape). Taking that off made installing the header easier. Also might help to remove the undertray for the front bumper. And only torque down the bolts to 10ft/lbs. And maybe in a day or two torque them down again.