I'm not familiar with the question of load capacity for expo use, but springs are only one part of load carrying capability. I've worn out several (6-8, maybe 10?) axle shafts in 1/2 tons and light duty 3/4 tons. Heavy loads are hard on them. Worse case, they break, shearing the axle, and the wheel taking off on an adventure of its own, and you leaving a trail of sparks and loud scraping noises.
The issue with most 1/2 tons is the bearings ride on the shaft itself, wearing the shaft. The older 3/4 tons, and some of the older Ford 1/2 tons had separate bearings and races, when they wore, the parts could be replaced without replacing the axle shaft. They also had larger bearing surface areas than 1/2 tons. I think some of the Jap trucks had better bearing types also, but have not been in them.