You’ll likely be over the GVWR, but not over the rear axle rating and tire rating as long as you have the rear tires near the max. I’m assuming that will be 80 lbs. I think you’ll be good to go to try it.
Last time I checked, besides badging, the difference between the F250 and F350 is the block under the springs in the rear and the rated capacity. You may need airbags, stableloads, or timbrens to level the vehicle. With airbags, I’d also add extended bump stops of some type. The difference is likely going to show up unloaded more than loaded. For that reason, I’d try it stock, first.