Eaton Detroit Springs...
http://www.eatondetroitspring.com/findmyvehicle/
shows the heaviest coil springs are for a 402 for the 1/2 ton 2wd and two leaf rates (440 $ 660 lb/in) for the 4x4 (350 cuin only). The coils are for "with AC or without" these would be the best right off the shelf for coils and the higher rate leafs will be the best off the shelf. You can even fanagle the ride heights, a 1" higher ride height spring may work under your heavier load to have the same ride hieght compared to the original height, you never really know until you try.
Now, I have worked with them before and they will custom make your spings to be more heavy duty if you care to go that way for a bit of extra cash. I would give them a call and at least learn from them what they recommend. You will have to give them some engine/drive train weights to work with (before and after the mod).
Springs can be a lesson in patience because messing with the front springs may/will change your rake and you may then have to change your rears and it rarely goes without some trial and error. I typically end up with an extra new set of springs to trade off.