My '50 half ton has overload springs on it too, over the axle, with rubber bumpers to contact the frame. They look stock. The rest of the truck has the stock parts. It does seem to work like a heavy half, although the overloads don't kick in until the regular springs drop enough so they hit the frame.
The reason your springs are really stiff is that they have rusted together. If you take them apart, clean up the surfaces so they slide smoothly, grind off any ridges, and replace and grease the bushings, the ride should soften up quite a bit.
