The GT500 mufflers are some of the best you can get, they are considered an upgrade for GTs. To make it louder, Dynatech Longtubes/mid pipe and delete any resonators. Leave the mufflers alone though, they sound amazing as is and with a better flowing setup into them they sound even better. You don't really need much on those cars power wise tho, much better to spend money making the thing hook up.
Also, THE very first mod for a GT500 pretty much needs to be a MGW Gen 2 GT500 shifter. The stock shifters in those are so bad that you can actually damage the trans over time if you do any sort of "spirited" shifting.
Also, sick ride.
2000 Mustang GT Steeda #0048
Just because I give you advice, doesn't mean I know more than you. Its just means I've done more stupid ****.