What you need is a progressive rate spring, which is easy enough to build with leaf springs but I don't know if anyone sells it off the shelf. The way it works is that the leaves are not all arched to the same radius. As you go down through the pack, the lower leraves are arched to a larger radius so that at normal ride height and normal load, they either don't contact the leaf above, or if the pack is clamped together they are actually being pulled up rather than being bent down by the load. With a spring built this way, as you add load and the suspension settles, it progressively engages more and more leaves. I would think such a spring might call for a minimum of 6 leaves, with the top three or four being normally arched and the bottom two or three being progressive, so that under normal conditions they don't do any work.
An extreme example of this is the short, thick, stiff, flat overload leaf found under Comanches and many other pickups.
I think Goatman runs rear springs with a boatload of leaves. Check with him for advice on this.
