I just bought a brand new 5th gen 3500 SRW 6.4L Laramie with 6'4" Rambox bed, trading in my 2013 1500 Laramie with the 5'7" bed and ramboxes. Can any of you with a 4th gen 2500/3500 and ramboxes post your dimensions of the bed? Most of the threads and google searches I have found are for the 1500 5'7" bed and I just want to make sure the available tonneaus for the 4th gens will fit. I will post the dimensions of mine here in a bit. What I am looking for is rail to rail sideways (front and back) and bulkhead to tailgate lengthwise, if ya'll could help-a-brother out. 
I have 74" long from the inside of the bulkhead to the tailgate (on top of the cargo management rails) and 53" from box lid to box lid.

I have 74" long from the inside of the bulkhead to the tailgate (on top of the cargo management rails) and 53" from box lid to box lid.