Not an expert on this subject but what I'm understand about the F-350 SRW, is they have what's called a "Top mounted over load spring(s).
When LOADED, doesn't the top mounted over load spring come into play?
Will it stiffen your "unloaded" ride if you add Sumospring(s)?
Have you talked to your Ford parts department or your local "Truck Spring shop" for better overload springs?
I pull a fully loaded F-250. Mt local truck spring shop installed extra leaf and Timbrens. Yes I knew it would stiffened my ride. I now have over 12,000 on my NP381 and its working. I might consider a conversion to the top mounted over load next year.