There is a point where Ford began making two F-250 models, a standard and heavy duty version. I don't recall what year this took place, but you may want to check which you have. Look at your rear wheel without a hub cap on at the axle in the center of the wheel. If it protrudes through the wheel you have the heavy duty version, if the axle is flat behind the wheel, you don't. If you don't, use caution in beefing up your suspension, your axle will be the weak link. If you have the heavy axle you could consider putting on the helper springs and main spring pack from the F-350 to help out.
Your local heavy duty truck repair (that does suspension) can Re-Arch those rear springs, it will help keep the cost down rather then replace! 2nd go to NAPA and buy there version of rear load boosters, around $100.00 20 minute install and get 1000lbs extra haul capacity or more!