look up any of the AM performance stores that are W- body specific and price out a whole DP/cat setup, should run you around 250-300 for the whole thing. then you'll need new gaskets for it, but after you put it on, you'll notice a difference in throttle response and you should end up with between 1 & 3 MPG over stock, due to the increased flow (that's what I ended up with when I put my JimmyC on)
www.zzperformance.com www.wbodystore.com www.grandprixstore.com check these guys out also :
www.hoganperformance.com I'm sure that if I missed something, someone else will chime in
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98 GTP GMPP handling kit, HomemadeFWI, MSD coils, Taylor wires, 180 drilled tstat, TR6IX plugs, Intense STD PCM, JimmyC DP, Thrasher shift kit (mild setting), AGX/Intrigue spring setup, drilled/slotted rotors with Hawk HPS pads. Hopefully more to come