don't forget shocks too
Don't forget to go with heavier shocks also, most likely dual shocks. The spring rate really only sets the ride hight of the truck. Resistance to bottoming is mostly dependent on your shocks. I've learned a lot about suspension from my motocross racing experience! In motocross you set your ride height with springs. Everything else is relies on your shock dampening characteristics (compression & rebound). Basically if you go off a big jump and your susension bottoms out, it's because you need more compression dampening, it's not because your springs are too soft.
The same thing applies to trucks. So, have some custom springs made to get the correct ride height out of your truck, then look into some good heavy duty shocks, and you'll be set!