Leaf springs for dart swingers.

That is true. Look at your springs though. Even with the shackles and the helper springs the main leaf is still flat. It is supposed to have some arch. If you add a leaf I am afraid it will be like putting lipstick on a pig. I would just put a new set on. They will outlast the car.
In that photo I posted those springs was brand new then I had just put them on that day the pic was taken and they was flat. Even back then I knew that wasnt good thats why I added the helper springs. I wish I would have known what I knew know and just got the good 6 leaf springs back then.