I guess I'm more curious if you are concerned with volume, or weight. They will haul way more than the rated GVW. Now if you have an accident, and deemed your fault because maybe you could not get stopped in ti,e, because of being overloaded could be a different issue. This is my '93 Chevy I bought new. Most generally, when hauling limestone, I haul around 3.5 tons. Here is a picture of where I'm hauling from a gravel bank some neighbors have, and let me haul out of. I load it myself. I've hauled enough on it, and have had 4 tons on it, and this is what 4 tons looks like on it. I've hauled more than 1,000 loads out of that gravel bank like that, to build pads for my new buildings, in the last 6 years. The great thing is, it's a 4 mile round trip. The first 3800 tons I hauled, they charged me $1 per ton. I can make 32 rounds on a tank of gas. At that time, it took approx. $32 to fill the gas tank. Figuring in the fuel for the backhoe, and wear and tear on the truck, my cost was still under $2.50 per ton to haul. You can't buy squat for $2.50 per ton, here. I've hauled enough on it, I've literally worn holes through the bed, that I've had to weld shut, and still do occasionally. Dumpy has definitely earned his keep.
If the F-350 has something like a 10' flat, with dump, I'd say you'd never overload it. Legally, yes, for how it's built to carry, no way. I'd be more concerned with volume, than weight, hauling horse manure. Stone, or gravel is another story. Just hold up how many fingers, for the amount of tons you want, when the loader man looks at you. Unless they have really old equipment where you get it, most newer loaders have scales built in, so they can come pretty close, and usually a little light. Put some good 8 ply tires on it, and keep the brakes up to snuff, to get you stopped in case of a panic stop. I'd say you'll be good.
For the ZF transmission tail shaft housing designed to leak, you'd have to go ask the engineer(s) that designed it, as to why. Could be, they were a descendant of the engineer that designed the Detroit Diesel, 318 slobber box.
As for the insurance, could be insurance companies in different states, have different policies covering small dump trucks. I'm guessing weekend warriors having small trucks, hauling loads to do driveways, and tearing down phone lines, etc, have a lot to do with it.