Like everyone said, cold setting a steel frame or just pulling the rear dropouts a little further apart to shove a 5 mm wider hub in is no big deal. That's one of the neat things about steel.
If you're worried about spoke breakage, build the wheel with top quality butted spokes, fit the widest tires you can, and keep the luggage weight as low as you can. You don't say how tall you are or what kind of physical condition you're in, but most 200# people could stand to shed a few of those pounds, further reducing stress on the whole bike.