You still haven't answered how you are going to use the truck.
My 1959 1/2 ton longbed gmc has a set of helper springs, or overload springs on it. It also has air shocks. You could increase load carrying capacity in those simple ways. There is also a load handeling air bag available that can help carry a load.
As stated above if you want to carry more weight, it is important to be able to stop more weight. A disc brake setup on the front will help. The rear only accounts for about 15% of the total braking.
