Truth be told I got my record mileage through the $55.00 three inch, 3 stage modern Catco cat.
Going the way GT Rider is suggesting is a sure fire method to better mileage and a serious low end torque increase. The real beauty is you don't need the headers to use the Y-pipe, so you can do them later (if at all).
TRH recently went Bassani and the so called "Sixlitre" tune up and he went from 10 mpg to 16.8 mpg with just those two changes.
Whats more he travels most places at 75-80 mph.
Duals are more about top end power and trucks just don't need the gain in the upper rpm range, they need it lower.
If you're after an aggressive sounding truck, just don't include the cat (or use a hollow one like me

) and you'll have trouble muffling her down quiet enough for your liking.
Spend the big money on the Bassani. After that we've found a way to economize on the tailpipe with Maremont offering a 3" mandrel bent unit for as low as $23.00.
Then you pick any 3" muffler you want and bolt it all up. Like GT says many of us have discovered the Areochamber as a way to have your noise without any drone at cruise (like you know who).
Sixlitre