I didn't like the sound of the stock GT muffler, so when it fell off and left me with an open exhaust, I wasn't too upset. But eventually my neighbors commented, so I spent an afternoon putting a "Turbo" muffler on. I paid $20 for the muffler from Auto-Zone for their smallest muffler, which has a 2" pipe. I figured it wouldn't be too much trouble to get a couple of couplers and then expand and reduce pipe diameters as necessary.
It came out terrible. I had to make a second trip back to the store for more reducers, and even then I had expand, reduce, trim, file and weld a great deal more than anticipated to get the 1.5" diameter stock pipe to mate with the 2" pipe on the muffler. I also had to push the muffler towards the rear of the car because the body of the muffler was hitting the passenger side bracket for the e-brake. Now it hits the spring cup, and doesn't leave me enough room to get an over-the-axle pipe in there. The sound isn't great, it's neither real quiet like a stock muffler, nor bravado like the open exhaust was.
Bottom line, unless you are going to go full custom, replace it with the stock components from OGTS. It will be a great deal easier to install, sound clean and quiet, and cheaper in the long run.
If you are interested in going with a non-stock setup, their is a great thread by "Bucky" and a few others. If I remember correctly, he even posted a sound clip of the finished product. A search will turn that up for you.