The below is a darn good question:
"Just which University granted your Mechanical Engineering degree and in what states do you carry P.E. registration?
This is important since you seem to know more about axle design than the Engineers working for the manufacturers."
To the OP: the question for you to decide is whether you want to stay within the ratings provided by the manufacturers or whether you want to go with the opinions of a bunch of self-assured internet experts.
Those same experts will critique the GVWR and GCVWR provided by the manufacturer ("its no different than the 350" "Legally, it doesn't matter" "Axle ratings are all that matter"), but the only party that assembled and evaluated the entire truck is the manufacturer.
Again, your call if you want to remain within the manufacturer's ratings or if you want to ignore them. When I towed my 5th wheel, I felt it as an acceptable risk to be 500#'s over the GVWR (on a 13,000 # GVWR) since I was about 3,000#s under the GCVWR and since that 500# overage only came if I had full water tanks plus everything else as loaded for a month-long trip.