Noise laws don't restrict equipment, they restrict noise. The Coast Guard restricts equipment. If it's OK by CG regs and the noise is below requirements, you're golden. But, the direction your headed is 180 degrees the wrong one. Those headers are for making power in race boats, with no consideration towards sound or legality. They are that length because there's no physical way to craft a water-cooled marine header and have equal size and flow runners and proper collector length and still dump horizontally through a transom.