You'll get a better seal on your muffler when using a composite gasket if you soak it in oil for a couple hours before installing it. After running the engine for a few minutes check to make sure the bolts are still tight. You might have to do this a couple times until the gasket compresses fully. Do not over torque the bolts. Use a lock washer if you can. If you over torque the bolts you'll take all the stretch out of them
Sometimes it's better not to use a gasket at all. Instead, clean both surfaces with alcohol or acetone and then put a very thin layer of Permatex Ultra Copper RTV (available at most auto parts stores) on both surfaces. Let it dry overnight, then, using an Xacto knife, remove any excess inside the exhaust port.Tighten the bolts with moderate torque Check tightness after the engine has warmed up.