the injectors for the 6.1L will fit, but they pump more fuel, so you will be running rich. You would likely need cams/tuning/possibly forced induction to take advantage. More fuel means you need more air. the difference in A/F Ratio from 5.7 to 6.1 is not much so you MIGHT be able to get away with just a tune.
But hey, I am always looking for a reason to BOOOOOOOOOOOST
The Leveling kit? I would go with Bilstein 5100 Shocks, and call it a day. They are adjustable shocks that you can raise to level out the front of the truck. It used to be the case back in the old days that leveling kits caused premature wear, not so much anymore. And even so, with the Bilstein's, its as easy as twisting the adjustment nut to lower the ram back to stock. Thats my only complaint about these trucks is how low to the ground they sit. You cant do ANY kind of even semi-serious off roading without some sort of lift.