... it seems that the f450 has a lower payload than the 350 ...
2021 F-350 DRW pickup trucks and 2021 F-450 DRW pickup trucks have a GVWR of 14,000 pounds in the US version and 6350 Kg in the Canadian version (about 13,970 pounds).
Anything added to the truck cuts in to potential payload (because of the fixed GVWR). Super Cab instead of Regular Cab: less payload. Crew Cab: even less payload. Diesel instead of gas: less payload. F-450 pickup with heavier parts: less payload than a F-350. 4x4 has less payload than a 4x2.
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2017+ F-350 pickup trucks and 2017+ F-450 pickup trucks have a fully boxed frame which have greater stiffness than chassis trucks of the same years and models.
This means that the above pickup truck frames will twist less than the chassis trucks.
You will likely want to build your camper on a pickup truck to minimize flex in your camper.
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It is probably easier for you to insure a pickup truck versus a chassis truck. The pickup truck with bed is a "completed vehicle" in the US. Does anyone know how Canada does this?
.... Why are decisions so hard?! Is it cause I’m a woman?? ...
Nope!
It is because you are still figuring out what you want to do, which means you are thinking about things, which is a good thing to do!
...You are correct that I don’t know exactly what my camper build will (weigh), the lovely couple from Ambition Strikes built their camper 8 feet long and it weighs 4000 wet with their dogs etc. My camper will be quite similar, 9 feet long though so I am just sticking with the 4000lb number for estimation sake. As I build with a focus on lightweight materials and minimalism if it comes in under weight then there’s more room for toys.
... I am going to meet with Ford about getting a gas 7.3, but I know my dad wants me to get the DRW for better stability. ...
Before ordering a vehicle, get a printout of the build with the options you have chosen to confirm payload, especially if you order a SRW.
How wide a camper box? How tall? Fixed hard walls? Popup with hard walls? Popup with soft walls? Cabover section? If so, how long and how high?
Regular Cab, Super Cab, or Crew Cab? Long Bed or Short Bed?
SRW or DRW? The DRW will help with stability, especially if your camper is tall and wide (consider strong cross-blowing and gusting winds while driving (and camping).
... But I went back and looked and unless I missed it, you never said exactly what you plan to use this rig for. Full time living in the back country? A few camping trips per year, and staying in campgrounds? Just as a fun project? Just yourself... or how many? What sort of amenities and capabilities do you want? etc
And the above questions too.