There really isn't any "GT kit" available for your V6 suspension, since the only changes on the base GT are higher-rate front springs (due to the heavier engine) and a rear sway bar (the V6 doesn't come with one).
If you want to improve your car's handling, I recommend the Bullitt package. It's available from a number of sources, and includes front and rear sway bars (stiffer than the stock GT bars) and firmer struts, as well as the Ford Racing springs that will drop the car about 1/2". If you don't want to replace your struts and/or already have lowering springs, buying the Bullitt sway bars alone will greatly improve your handling.
EVERY V6 owner should install a rear sway bar of some kind! It was a mistake for Ford to delete it from the V6 for cost savings, since the car absolutely needs it for any degree of performance driving.