When you get into medium duty trucks there are literally hundreds of option combinations even on a given model, and as such GVWR's can vary a lot.
We have a '75 C60 that is setup for maximum payload, but only has a 350 engine and 4spd transmission. It's double framed, 23,000lb rear axle (2spd) and 9,000lb front axle, however the GVWR on it is only 24,000lbs due to the tires, which are 9.00-20. It also has a pretty heavy duty twin telescopic cylinder hoist (not scissor hoist) despite having a 14' grain bed.
Basically the truck itself will haul more than the tires or grain bed are designed to. It has farm plates on it now, but used to be licensed at 32,000lbs and would gross every bit of that fully loaded. I did load it to capacity once, we were loading shale on it and I decided to see how much it would take to set it down on the overload springs. We had the 14' grain box almost full to the top with shale and it was just on the overloads. I don't have any idea how much weight that was, but the truck hauled it fine, the hoist lifted it fine, but the bed was very wobbly and creaky when dumping it.
If you are going to haul with a grain bed, make sure the floor/supports are good around the hoist (I have seen the cylinder poke right through the floor), don't haul wet sticky stuff like clay, and don't overload it. Keep the load centered and only dump on flat ground. Being on a little bit of a side hill can twist the bed right off the truck, or just turn the whole rig over.
I do use ours to haul gravel/dirt some, but mostly around the farm or a job site. Over the road it's generally cheaper to call the quarry and have them deliver.