I have a 78 GMC 3/4 ton 4x4 that I use for a service truck.  It has a steel flatbed and toolboxes etc. which make it pretty heavy.  I would like to put a 2" full kit with new rear springs because my rear springs are shot.  I would like to maintain or increase the load capacity of the rear springs since I am carrying around a lot of weight and I also carry around heavy stuff (up to 3000lbs) on the flatbed at times.  I am looking at the Rough country kits, but I can't tell by any of their literature how the spring rate compares to a new stock spring.  Any tips or advice on how to go would be appreciated.  

thats a good way to go with the addition of a set of HD air bag helpers in the rear...It'll nearly maintain a level stance even fully loaded not to mention improved handleing.