Pontiac's mufflers were produced by Oldberg and were designed for a specific sound. Did your Grand Prix sound like this?
https://www.youtube.com/embed/U7l12H...feature=oembed
If so, you might consider this system from Gardner Exhaust. It's expensive, but well worth the price!
https://www.gardnerexhaust.com/exhau...aust-system-3/