A chip won't do much on a near stock 4.6... save your money.
Some states REQUIRE cats and mufflers for emissions/sound pollution issues, and to be honest, I don't think you'll gain much from just removing mufflers anyway.
Get yourself the full BBK H-pipe setup with high flow cats, and if you've got some more cash to spend, go the whole hog by adding a BBK cat-back system with performance mufflers too...
That way, you'll be happy, and so will the state
Also, don't forget some pulleys as a possible mod too...
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98 GT
Bassani X-pipe, Dyno-Max Cat backs, 3.73 gears, Steeda shifter, FMS pullies, 87mm MAF, Roush cold air kit, Eibach springs, and Zex 75 shot