The other option is the Bliss Race Engineering (BRE) box muffler. This is a INEX approved muffler and comes with the INEX stamping. It fits under the left running board, and makes the car sound like a street bike. We run several endurance road races a year that go into nightfall, and the decibel reading(s) have never been an issue. The muffler is designed to not hinder performance and they offer documentation of the differing horsepower values by all INEX approved header/muffler systems.
Call BRE; Mike @ (209)892-2543
Dave @ (707)462-9256