Congrats on gettin' a new Stang. First, it depends on what you want your Stang to sound like. A catback alone is only going to net you 1-2 hp if that. It's basically only a sound mod. It's all part of the mods you want to do though when opening up the whole system. In essence an Hpipe will give you more torque and an Xpipe more topend (in essence). So it depends on what your ultimate demands for the car will include. If you are considereing pipes without cats altogether you will need MIL eliminators (message indicator light) so your computer won't have fits. Most people don't fool with headers on the 4.6's as they are a b*tch to install and unless you're planning on gobs of hp from other mods the gains can be had from other boltons.
Others will chime in but I hope this sheds a little light.
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66 Tahoe Turquoise Coupe
Laser Red 96GT Ragtop
30 Years & 30 inches apart in the driveway!