Your truck should look about like this:
This is my wife's Tahoe:
It's not the best picture, but you can see that the air dam hangs down a few inches lower than your truck does. You would probably need the air dam and the lower part of the bumper that it attaches to, but I'm not 100% sure.
The e-fans are supposed to be good for a .5-1mpg improvement on these trucks. My wife's Tahoe has the same 4.8L/auto drivetrain that my Silverado has, but the Tahoe will get 25hwy with her driving it. At best, I can get about 23 in mine, and that's using some hypermiling techniques, it would be more like 20-21 driving the way she does. Her's has the air dam, e-fans, and slightly higher gears, plus it's been lowered about two inches.
I switched from steel wheels to these aluminum wheels:
And saw a slight increase in economy. Just watch what tires are on them, when I switched to (heavier) LT tires it came back down some.