making sound

I am looking for advice on exhaust systems for GT1 car. Having a very hard time making sound requirements. Have tried several mufflers and combinations of baffles placed at various points in 4 inch exhaust.I am a new member of crew working on an 06 corvette built as a trans-am series car racing in GT1 class. About 750 hp v-8. We need to be less than 103 db.