[old Chevy guy]

How many leafs do you have now? I added three extra leafs into the stack on my 71 C 20 and it stiffened it right up. Haven't had a huge load on yet since then but had 2300 pounds of roof shingle tear offs and it barely felt it. I bought new u bolts and center bolts from the local spring shop and put some leafs I had from some parts vehicles in. All the leafs original and the added ones were in good shape though. Right now I have 8 leafs plus the heavy overload leaf on the bottom. The spring shop says you can estimate adding 500 lbs of capacity for every leaf added to the stack. So I figure 3 leafs per side added 1500. Of course springs are just part of it ,you have to remember the brakes and the frame remain the same, it just keeps it from squatting quite so much.