how big of a tire can i fit on a 17 by 9 rim,also what can i do to upgrade the braking system that would be worth it

 

Following most tire manufacturer's recommendations, the largest tire you can put on a 17x9" wheel is a 275/40-17. Some people go to 285/40-17 with no problems, and there is a thread here showing some 315/35-17 Nitto Drag Radials on a wheel that narrow, although the manufacturer does not recomment it.

As far as a brake upgrade, the most cost effective move is to swap in a set of 98-02 brakes in the front. They are larger than the brakes use on the LT1 (93-97) cars. Baer and Wilwood also make upgrade kits for the 4th Gens. Best place to get advice on a brake swap would be on the "Suspension, Chassis and Brakes" forum.