'99 F250's have 6084 lbs RGAWR, due to the 235/85-16E tires and 16x6 factory wheels and factory rear spring packs, without upper overload springs.
'99 F350's with single rear wheels have 6830 lbs RGAWR, due to the 265/75-16E tires and 16x7 factory wheels, with the same factory rear spring packs as the F250's, but with the upper overload springs as standard equipment.
Some F250's came with 265's and 16x7's as a factory option, as well as upper overload springs as factory option (springs included in the camper package).
Unfortunately, Ford did not change the RGAWR for those F250's equipped with the F350 tires, wheels and springs.
If your F250 is so equipped with the F350 tires, wheels and springs, you can safely load it to the 6830 lb RGAWR of the F350.
If it does not have these things, don't load it that weight, because you will be overloading the tires, wheels and springs.
You can certainly upgrade your truck with those parts and happily load it to that weight.