40s are going to kill any power that it has, there is quite a bit more rolling mass to them over 37s. If I were towing I'd be at about 6" or just a 350 w/ leveling kit and 35s. It's a nice looking truck don't get me wrong but you have to decide if you want to tow or show, I notice a difference from 33s to 35s on my friends CTD and he's laying down 1000 ft lbs of torque w/ 4.10 gears and a hand shaker. A 14" drop with that enclosed trailer is risky IMO. A little sway with anything over 8K lbs and it'll snap that CNC drop in 1/2 like nothing (seen em bend many times in my business). I've towed a lot and anything over 35s puts the powerstroke under much stress w/ it's 3.73 gears. What gears are you running? What is the weight of the trailer? How far do you tow? This will help answer your question.