Bandit, if you are looking to get the most mileage out of your truck, then here is a list of stuff that you will want to consider. If it says anything, I am still pulling 13 MPG running 37" mud tires with 4.56 gears.
The big one is the Gotts mod using a cotton gauze K&N filter. Lets the engine breathe better and also nets you 1 hp and 10 ft-lbs of torque (dyno numbers, ran on my truck). Next would be the e-fans. While this seems obscure, you are looking at gaining roughly 6 hp and 8 ft-lbs of torque. The beauty of the e-fans is that you gain that at both 500 RPM and 5,000 RPM. Not many mods give you gains at both ends of the spectrum.
The other mod that I would tell you to look into is porting out the 90 degree elbow just downstream of the throttlebody. Not that difficult to modify and clean up, but you can gain a little bit more there.
As for the exhaust, this is a slippery slope. For mileage, in short, the more restrictive the exhaust, the better the mileage. But at the same time, opening up the exhaust can get you more power at higher RPMs. From my playing and talking with others that have modified their exhausts, the breakover point seems to be something right around a true dual 2.5" exhaust setup or a single 3" setup, both running something along the lines of a Flowmaster 70 series muffler (fairly restrictive, yet flows better than the factory muffler).
Another thing to maximize mileage is to toss all the body bolt ons. Things like a brush guard, visor, window vents, etc add more drag to the truck, making it harder to push through the air. About the only bolt on that even remotely improves anything is the wing on the back of the truck that helps to direct the wind down over the tailgate. But, now you are looking at something that is taking the weight off of the rear tires and can make the back end of the truck hydroplane easier when at speed.
Another biggie is keeping the engine RPMs between 1800 and 2200 RPM. Getting below 1800 RPM and the engine drops out of its power band and the valve timing isn't quite up to *****. Getting above 2200 RPM and now you are getting into the major power producing range of the motor and when you make power, you are also using more gas.
Hopefully this gives you something to start with. I went one step further with my truck and ported/polished the whole intake system to allow the air to move easier. But, this was due to my background in fluid dynamics and me trying to see just how much of a difference it made. Don't pull the motor apart just to do this. But, if you have the motor apart for something else, by all means as it only takes a few hours to do and you do see a little bit of a gain. I had my truck torn apart to Timesert all 8 spark plug holes.
