Super stock springs are not handling springs. Your car will handle corners much better with HD rear leaf springs. "64" suspension is the same as any "A" body, pick one and go with it. I used to tell parts guys I was working on a 71 340 Duster.
I am well aware about the super stock springs. At the time that I purchased them I had read they were good to go with. They will be up for sale in the near future. I'm still curious to see how the car will look with them on. I'm not building a pure handling car, or a pure drag car. I want something I can cruise around in and not have the soft suspension feel.