You can go to 285/70/17" with no problem,,,,only issue maybe 285's seem to be only D load range tires for some reason,,,I have a set of Hankook Dyna-Pro MT 285/70/17" on my 06' F-350,,studded,,,for winter driving,,Awsome traction(just put them on yesterday,,pics in gallery)in mud and snow,,,I push a 9' Fisher plow,,,sometimes I have to drive up on Curbs, to plow a sidewalk,,,these tires have held up well,,,only being load range D,,,,I do not however carry a sander in my bed,,so the weight is not an issue,,,I do however have sand,(in buckets),,tools,,gas/diesel, etc,,in the Cap on my truck,,which normally weighs around 7800 lbs without the plow or sand.
I went with the 17" tires, purchased a set of 07' F-250 wheels,,takeoffs,(they fit the F-350 no problem),,Because the 18" tires are Big $$$,,and I did'nt really find any tread I liked,,,Agreed the Continental tires,,,just dont cut it for anything but a dry road!!