I have Auto-X'd on occasion although I am a Bracket Racer at heart.
That being said (that the advice is coming from a straight line guy) you could start here.
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SLP Lowering Springs"
An SLP exclusive, the progressive-rate spring package yields noticeably sharpened handling characteristics without an accompanying degradation in ride quality. Ride height is reduced by one inch, providing both a more aggressive stance and a lower center of gravity. Kit includes two front and two rear springs.
NOTES: This package has been designed to work with STOCK-SIZE wheels and tires (245/45-17 on 17 x 8) and may cause tire-to-body contact if installed on a vehicle with a larger wheel/tire combination. Professional installation and subsequent four-wheel alignment strongly recommended. Camber kit #70027 required to maintain proper rear camber.
SLP Camber kit
"Use this kit with our #70026 GTO Performance Spring Package to maintain proper rear camber. Fully adjustable, it allows you to tailor vehicle camber settings to suit various suspension configurations and performance requirements.
Part Number Item Price
70027 SLP GTO camber kit $159.99
SLP Heavy duty rear swaybar
New from SLP, this heavy-duty rear sway bar is an easy, cost-effective way to improve the traction and handling characteristics of any 2004 GTO. Constructed from 19.05mm (stock is 16mm) cold-drawn steel, it can yield noticeable reductions in both wheel hop and rear body roll during spirited driving. Two end-link mounting points on each side make it possible to preload the rear suspension for improved launches in drag-race applications. Includes low-deflection K-prene bushings.
NOTES: Not compatible with SLP #73005 Rear Sway-Bar Bushings.
Part Number Item Price
73004 SLP GTO Heavy duty rear swaybar $149.99
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Taken from the suspension section of this page:
http://www.tbyrnemotorsports.com/gtointro.html