Chambered Mufflers like the Flowmaster 40 have a great tone but they dont flow all that well. In some cases they have actually cost power over well designed factory mufflers on performance cars. (On a truck I would expect you might see a minimal gain.)
If power is your primary concern you want a straight through design.
I have the single in/dual out on my 02 4.7. Great sound, and I dumped it before the rear axle, so it sounds meaner now. You wont see any horsepower gains w/just a 40 installed, but you might get a bit if you add an intake, tuner, and headers.