The first thing I would do is chassis bracing. Maximum Motorsports, Griggs, Kenny Brown, ect. Subframe connectors at least, other braces if your budget allows. A good set of subframe connecters will run $100-$150, and any number of places can weld them in (don't just bolt them on) for you for pretty cheap. Just make sure the weight of the car is on the tires when they do.
Suspension would be next. A blown 99 GT will be making a goodly amount of torque. The stock control arms and bushings wouldn't brake very easily, but they will deflect all over the place, producing wheel hop, excessive spin, and just plain hard to control. You have two basic choices:
1. Replace your control arms and bushings with heavy duty ones, such as Steeda.
2. Replace the 4 link with something else. Griggs makes 3 link suspensions which would work extremely well for you. Both alternatives would work extremely well for you, but I would go one or the other before launching a heavy supercharged convertable on drag radials or slicks.
Here the addresses:
http://www.steeda.com/
http://www.maximummotorsports.com/
http://www.griggsracing.com/
Considering the power you're making and the weight of your car, I think some upgrade is in order. But like Hammer said, the most important part is the one behind the wheel. But have fun in that beast!
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AIM: Some97Snake