sick with the stock HD clamps, they are by far the best on the market, torque to 45-60 lbs (per the manual). don't use the ones that came with the mufflers or things will leak. they love to leak there
Pick up copper permatex at your local auto supply store and coat the inside of the muffler where it connects to the header, that along with the clamps will stop any leaks, it's not permanent, but it will work.