I think I remember seeing "helper" leaf springs that were basically half a leaf spring that bolted to the frame behind the rear axle, but didn't normally interact with the rear suspension.
When the load exceeded a certain level, the frame lowered enough that the spring contacted the trailing arm and provided additional support.
Does anyone else remember these?
Those so-called "helper" leafs, have come on every 60-something C10, and C20 I've ever owned. They're half-leafs, or progressive-leafs. Anyway. They make NO interference with your ride. It rides exactly the same loaded, or empty. I can't say enough (good) about 'em. By all means, if you can get a pair of them. Do it. You'll never regret it!