i think you should be able to go to a bigger bolt, you might have to drill out your stock parts to take the larger u bolt, but make sure there is enough left to be strong. you could have a custom spring plate made with larger holes in it. any good spring shop should be able to make any u bolt you could want. i have broke several u bolts on my excursion, and in my truck it really sucks.
Any spring shop should be able to make a custom set for you. A shop by my house charged 35 bucks for all 4 with lock washers and nuts. I upgraded to 5/8" u-bolts when I swapped an Explorer rearend in my Ranger. All you need to provide is the diameter of your axle tube and the bolt length needed, and they can make them right on the spot. We had to bore the holes on the spring plates larger to fit them. Deaver may have the ability to make them as well. If not, check your phone book for local spring shops.