I have the B&W Companion hitch in my 2013 Ram 3500, I don't have the Mega Cab but I do have the Crew Cab with long box and DRW's. I set mine up with the hitch directly over the Axle and on the Shortest height, these trucks sit pretty tall in the back end.
You don't say, or I didn't see it, but so you have a short box? if so you might have to take in consideration the corners of your 5er hitting your cab if the hitch is to far forward.
by the way I love the hitch, yes it is heavy to quickly remove but it is one nice hitch. I had a Moryde pin box installed on the trailer and thats a nice improvement as well.
Hope this helps some.
2016 Vengeance 3062V
2013 Ram 3500 4x4 CC DRW 6.7L
2015 Harley CVO Limited
2015 Harley CVO StreetGlide