I don't have a dually, just a 3500 SRW 06 with overloads.
does you backend drop 2" before it hits the overloads? When I put my 5er on, it was dropping those 2" plus about another 1" of loading the overload springs.
Anyway, I put these new rubber bushings on the overload perches. The old ones were around 3/4" and the new ones are 2 5/8". While that sounds like a lot of difference, when you start to load the truck these 2 5/8" bushings have holes in them so they start to squash. End result is the truck don't drop as far when loaded (now its level loaded).
An extra benefit is when loaded to the point where your springs used to hit the perches and make noise, now the new bushings are somewhere between "full sized" and fully squashed so you never feel the helper springs kick in.
Empty I don't notice any difference so I just leave them on all the time.