I don't have a spec sheet handy, but you may find that the 3/4 ton springs are longer and wider than the 1/2 ton, which means you'll either have to fabricate mounting pads on the axle or use a 3/4 ton rear end... which limits your choices of gear ratios. I've heard of people sliding the center leaf out of a pack to soften the ride, but I'm not sure if you want to pull two out of each side. That might be enough to make it sloppy if the spring pack is long, and will certainly put more stress on the ends of the main leaves.
