The chassis cab trucks are different animals than the trucks that come with a pickup bed from the factory. Personally, I like the looks of the pickup bed better than a flatbed, plus I have no use for a flatbed. I also wanted a King Ranch. Chassis cabs don't come in King Ranch or Platinum trims. The highest trim level you can get on a cab and chassis is Lariat, which can of course can be optioned out pretty nicely. King Ranch trucks are actually Lariats with the KR package added on top. Also, after four years of personal experience with the 2010 F450, I knew a NEW one would handle my camper just fine with minimal suspension mod's required. I would NOT have bought one of the 2011-2014 F450's because of the changes Ford made to them in those years.
The rear axle is a Dana S110, and is rated at about 14.7K by the manufacturer, so I don't think I have to worry about overloading the axle itself. It's also not available in the F350. The wide-track front axle is standard on the F450's, and was a must-have for me. The turning radius with that axle is awesome, and has been very much appreciated by me several times. The wide-track front axle is only available on the F350 if the correct option package is selected, and you rarely see it selected. I'm not concerned about the weight rating of the front axle since I know my TC doesn't put much weight at all on the front.
The rear springs are what they are. I think Ford choose to give "ride comfort" a higher priority over payload. I used some home-made stableloads on my old truck, and was happy with the results. That was the only suspension mod I made to it, and mine weren't even half as thick as Torklift's. The only thing I wasn't satisfied with about my stableloads is they were just as noisy as the OEM rubber pads, especially when they were dirty from running down a dusty road. I was hoping they would be quieter since I made them out of UHDP plastic, but they made a lot of noise too. If Torklift stableloads don't do what I want, I'll take the truck to a spring shop and have them add a couple of leaves to the overload packs, and that WILL take care of it once and for all. I don't want airbags or Timbrens, I want it to get on the overloads quickly.
The tires used on the F450 pickups are also used on the F450/F550 chassis-cabs. Per the sticker on my new truck, they are inflated to a max of 90 front/80 rear, so they are de-rating the tires a little right there. Are the polished wheels rated for the full 110 psi? I don't know for sure, but I suspect they are. I doubt Ford would put a 110 psi tire on a wheel that wasn't also rated for 110 psi, but I can safely say they are at least rated for 90 psi. The steel wheels are in fact rated for 115 psi, as I can see it stamped on the rim. At any rate, I could get a little more capacity out of the rear tires/wheels simply by inflating them to 90 psi.
An observation of mine that is totally anecdotal (which means it's meaningless) is that it seems that some owners of the 350 and 3500 class truck with higher payload ratings than the F450 were also needing to do suspension mod's to haul a heavy TC. I'm not going to go find any specific examples of this as I'm done with the research phase of my truck purchase. It was just something that I made mental note of once in a while. I'm now in the enjoyment phase and I intend to stay there!
:):)