[Sidekick]

If you want it quieter you have to go with smaller dia mufflers.

If you have a 427 you need a minimum of 3 inch dia mufflers 3.5 would be better (no bigger). With 500+ HP 3 inch work well but are loud. Superformance come with 2 inch int. dia. mufflers and they suck about 100 HP out of the motor at 6000 RPM.

Smaller is quieter....period. No packing or other BS internals are going to substantially make your car quieter. if you're will to give up some more HP go smaller!