You've got so much room under there.
Go custom, use Magnaflow and Vibrant.

That is a 14" long resonator. Magnaflow makes them in 22" as well.
See those long sections of tubing, cut and weld more straight flow mufflers in, such as the Magnaflow 4" round (OD not pipe diameter) and maybe see if you can add a Vibrant Ultraquiet resonator in there too.
Those who are running the BRM exhaust on their Nissan's will know exactly what I'm talking about.
One, or if possible two of these
http://www.magnaflow.com/02product/s...e=main&id=9253
And one of these
http://vibrantperformance.com/catalo...1022_1033_1055