dont tighten it so tight for one. you snug the bolt, then lock it in place with the nut. If the prop is ballanced, and the engine is mounted properly, there shouldnt be any problems.
david

Bignasdy is right. As the muffler heats up, it expands, and the screw might not be able to hold that much stress. If you've ever tightened a muffler before it cools off you'll know what I mean. Get it just tight enough to hold it together without any slop.

Locktite, or Permatex high temp gasket sealant to help prevent the nut from vibrating loose before the muffler expands from heat.