If you are not supporting the muffler, I would rig something to do so. Like Brett said, that's a lot of weight jumping around back there. That should cure the bolts coming loose. Any air exhaust for cooling should be right at the end of the muffler, and be a combine area bigger than any inlet. Air is a liquid and takes the path of least resistance. You can protect any of the structure with some common aluminum foil, but make sure you have a space between the muffler body and the structure,
Type at you later,
Dan McEntee