It would drive a math wizard nuts trying to figure spring rates for a car and be accurate. There are so many influencing factors, The best you can do is a good guess then adjust depending on what car handles or feels like.
I take it you have leaf springs? That is what a Ford Cobra was equipped with. There was no mention of front springs. You know that the rear and the front should be compatible if your going around corners fast. If they aren't you front don't want to turn or your rear will slide to the outside. Is the front an A/C control arm? Or a recruited ford? Do you have roll centers for your chassis? What do you expect from the car when done? All of these questions could effect what spring rates you want and more if any suspension parts or mounting points are changed.
If you just want a soft ride, get something from a spring manufacturer that is for a car of comparable weight. They rate springs for factory applications. from rear weights, I would guess around the weight of 65-66 mustang with 6 cylinder (pony car). If it's too stiff take out short leaf rear half. On a leaf spring the front half controls spring wrap up and the rear is for rate. As far as ride hight they can be re arched or de arched at about any good spring place that specializes in replacing them. You can do this yourself if you have a hydraulic press a big hammer and able to make a jig. If you have a Mustang two rear. I put one together just this way for my brother in law. cut rear of short spring and de arched to get ride hight. They felt a little stiff but matched up well with the front they where stock Mustang 2 spring with one coil cut. make sure you give it a good alignment after.