When I wanted to change to dual exhaust, I asked Ford Racing as well as muffler shops. I got some real good info, including some from our illustrius Todd Z. Bottom line from Ford Racing was that I could put true duals on it, but that would compromise the cat set up and possibly destroy the bottom end. One muffler shop said it was the thing to do, and another more honest one said I wouldn't gain anything with a stock 244 cid and that increasing the pipe from the last cat (the one without all the hook ups) from 21/4 to 21/2 or MAYBE 3". Todd helped me out and warned me that the inital bottom end may be affected. Here's what I did: I figured that I use my ST for occasional heavy hauling, long trips and heavy loads in the bed. The last thing I need is to sound like I'm cruising at 90 mph while pulling something in 1st gear. I eliminated the last cat, (that was one of Todds helpful ideas for me), kept the stock exhaust pipe size, eliminated the resonator and replaced the muffler with a magnaflow. That's all I need. I will eventually replace the some parts when I keep it strictly street, but that's much later.
Ed