First off, if you need any parts for the GT500 conversion let me know. I decided against mine, and have a few pieces laying around that are for sale.
As far as exhaust, I would go with ARH for headers, they are the best. Pick the muffler of your choice via youtube videos. My car sounds like the following:
As far as suspension, I think it depends on what your goals
are with the car. If you want it to be a 100% road course monster, go with griggs. If not, there are much more cost friendly options that are still high quality.