I've got Flowmaster 40 series mufflers on my Fifth Avenue with a 9.5:1, Eddie headed 360. The exhaust is 2.5" with cutouts under the doors. Honestly, I can't feel any bit of a difference in power with the car when going through the mufflers compared to the cutouts. The motor should be good (never had it on a dyno) for 380HP. Now the drone, well, that's another issue.