As an owner of a 2000 F350 Supercab 4X4 with the 7.3 I can offer an opinion. It should work OK for you.
Just got back from a 1200 mile trip hauling my 9300 GVW, got pretty good mileage, very little freeway travel. Couple of time in second gear going up hills.
Does this truck have the push button overdrive lockout like mine does? What's the transmission?
You might want to look at various fifth wheel pins, there isn't going to be much give in the truck, but a fifth wheel pin with some give in it might help too.
There are lots of larger rigs hauled by F550's. At 35 feet and 12,000 pounds, you will be well matched with extras in the club cab seats too.
The 7.3 is a great engine. Go for it!