The secret to any good seal is flat, true surfaces. Gaskets can only bridge so much gap. Make sure the head surfaces are clean, and flat. Use a steel scale or straight edge to check it. Dim the lights and put a small flashlight on the back side of the straight edge. If you can see light between the straight edge and the head it is not flat. This is usually not the problem. If it is use can try the trick below, but I would recommend a machine shop.
On the header you can try the straight edge. In most cases it is just as easy to get a large flat file and run it long ways back and forth on the mating surface of the header flange. You will then see if you have high and low spots. If you do, keep going until it is flat.
If you get both surfaces flat, you can almost use no gasket and it will seal.
I know this is not the most high tech way of doing this, but it has worked every time in my and alot of my friends cases. Good luck, I hope this helps.
