If you tow, I would advise against leveling the truck. It will make the back sag, that's why these trucks have a factory rake, so when you put a trailer behind them they sit level. I installed the Monroe sensatrac load leveling shocks. They have a coil spring around them and are a direct bolt in replacement for the lousy stock shocks. No change to unloaded ride, though they will lift the rear of the truck an inch or two.
The stock air system on these trucks is very good, you won't see any improvement by adding a K&N except a lighter wallet and the oil from the filter messes with the sensors in the intake. Also, leveling will reduce your mileage but so will the bigger tires so be prepared for that.