If you look at the piping diagrams for the birds, on some there is a resonator positioned aft of the x-member - inclined upward to cross over the axle. Cars without the resonators simply have a section of pipe in this position.This stripped frame shows the location well enough....

It only takes a little imagination to move the mufflers out from under your seat and place shorter cans in the resonator position. The forward pipes can be insulated with header wrap if nothing else - and the wall behind the seat can be nicely deadened and insulated.
Several who visit here have done this modification......