Quad exhaust on an inline 6? Probably sounds really cool, but what kind of mods to the engine (valve train and head specifically) have been done to support the increased flow rate? Less restriction is only good to a point then it affects the engines ability to scavenge the cylinders properly resulting in less power. Just look at all the folgers coffee fart can hondas that can barely get out of their own way. That kind of system will be big bucks no matter what 'cause you're talking cats too, and that gets a little tricky EPA wise (big fines and nasty things if they decide they don't like you). I hope you can pull it off though and post a sound file of it for us cause like I said, that's gotta sound cool.