The best way discussed thus far appears to be having the rings of the clamp over each side of the notch and then make sure it's cranked down very tightly. Alternately you can head to a parts store and buy some better muffler clamps, they don't cost much and are supposed to work better.
And they aren't overly loud. They sound great and at lower speeds I think are loud enough. On the highway they're rather quiet and in my mind I like that. If you want raucous they aren't the muffler for you.