Whole new world once you get over 200-220 bu, takes a good tire to handle the pressure you put on them with 250-300 bu.
I'v got some 220 bu (typically more like 200 actual) gravity boxes, and not so much stress. I've got a pair of 250 wagons (can get close to 300 on in good crop) and those tires are bigger, and still show more stress. Doesn't sound like much difference, but it is.
Typically a gravity wagon uses the same size tire all the way around, tho doesn't hurt much to have diferent on each axle.