I was looking into this as well and was told to use Flowmaster 50 series with Deltaflow. They are said to be not as loud and very low resonance inside the car. I'd like to hear them myself though. A local speedshop recommended them. For now I'm using Cherry bomb brand turbo mufflers, nice sound to them when you rev and under power but nearly stock at idle and cruise. I had them on a stock 340 YEARS ago.