I looked at a DRW 3500 on the dealers lot this evening and like d3500ram wrote, the 3500 has the frame mounted stops for the upper overload springs to contact and those stops are welded to the frame so a simple swap isn't really that simple. The lower overload spring on the 3500 is the same as the lower overload on my 2500 (maybe thicker though).
merc225, can you elaborate on what you found please? Even with the mods listed in my sig, I'd still like to improve the resistance to side to side sway since I want to pull a trailer. My mods have pretty much stopped the sway but I still think there would be room for improvement. I like having some safety margin.