I got my custom work done at Andy's Muffler in Snellville, GA. From Stone Mountain take 78 East to highway 124 in snellville. Take a left on 124 and go about a block. Andy's muffler is on the right. If you see the hifi buys, you've gone too far.
They built my custom y pipe, cut off my third cat and resonator, and installed custom pipe and a bullet muffler all for around $250. Maybe they can help you out too.