Coil-over Spring Rates

I have a '29 Model A Roadster that's about to become an AV8. I'll use a flathead, My question concerns spring rates.
I'm going to use a Heidts Superride IFS and a 4 Link rear with coil-overs.
What spring rates should I order?
This is all new to me.
Thanks,
Bob

 

so, what does Heidt's say to use? I had a model coupe A with rear coil overs and only way I ever found a way to get it to ride smooth was by adding weight to rear of car.

 

Flathead with Heidts IFS?

Interesting mix.

 

You need the unsprung weights and suspension travel numbers. Then it's just math. Have any of that?

 

For what it's worth the rear of my '32 "rpu" had 180 lb springs in it, 289 V8/C4. They were too light and I should have went with 225 lb.