!7" Ford rims are rated lower than their 18" cousins, so I'm not confident that just getting higher rated tires will be a safe choice. See if the dealer has "take offs" from a F350 that have the higher capacity. I was able to upgrade my F250 from 17" rims with 245/75R17 tires to 18" with 275/70R18 E rated 3640 lb tires for very little money. Later, I upgraded again to Vision 19.5" rims rated at 4500 lbs each and H rated 245/70R19.5 tires.
BTW: "take offs" are new tires and rims removed from a new truck because the dealer or customer upgraded the truck to an aftermarket set.