I just installed a flowmaster cat-back on my 98 Durango 5.9L. It was the truck/SUV kit with the 70 series muffler. It was $300 shipped from Prostreetonline.com.
Well, I'm used to imports, so when the first instruction was, take a hacksaw or sawsall and cut 2 1/4 inches before stock muffler to remove I was like, wtf? What's the purpose of buying a kit then?
The piping in the kit isn't any bigger than stock, and its not stainless steel, so I'd just buy the 70 series muffler, get a couple clamps, and do what I did for the flowmaster kit instructions for the lone muffler. Sawsalling upside-down is an experience.
Sound is good (I wanted quiet, but some performance) -- shows off my V8 at WOT. I don't need uber loud, if I did, I'd get in my Talon. =) They have louder mufflers tho. Performance has increased (slight) as well as gas mileage (also slight).