i would go with a MBRP kit... its one of the best fitting kits around and will retain your stock cat... their is also a optional cat and muffler delete pipe that can be bought seperately....for off-road use of course
Stealth Automotive. 2004-2007 Superduty 6.0L
the 03 turbo has a louder turbo whine which a lot of people like......
let me know if you have any other questions.. we can package something up for ya to save you some money