Putting a F450 or 550 rear axle, springs and dual rear wheels on a SRW F350 might get you out of one or two problems, but the door pillar sticker is still the door pillar sticker and that gets looked at by the folk in uniform.
You might be relatively safe to the rest of us, but still not safe from a whole list of potential fines and costs to have it properly moved from wherever you get stopped.
And that's just it. That's the bottom line. It doesn't matter if you put Mack Tandems under a 350 pickup bed, it's still only as strong as the "weakest link in the chain". It doesn't matter what brand of truck it is, if the O/p pulls the weight he says he wants to pull in post #1, he's over on GCWR and probably rear axle capacity and a few other infractions.
"Beefing up" the suspension may truly make the truck handle weight better, but all the state police care about is maximum allowable gross combined weight rating of the truck from the manufacturer. "Let me see your registration & registration" is how the conversation starts. Next is "your registration exceeds your truck manufacturer's maximum GCWR". Then it's "you trailer must be offloaded or abandoned until you are within legal operating limits, so I'm putting you out of service". Then comes the penalty/fine part of the discussion. Not fun.
The other thing to remember is it's not just exceeding GCWR, you can be well under GCWR and still have an overloaded rear axle b/c the weight is too far forward on the trailer, or you have too much cargo weight in the bed, cargo not secured properly, defective equipment, etc.
For RV's, horse trailers & boats, it's simple-anybody is allowed to do basically whatever they want. For commercial towing, you need training, licensing, insurance, med cards, DOT #'s, inspections and be in compliance of weight limits & safety regulations or it's just a matter of time you'll be paying fines.
