What I would do is make up some lengths of tube the correct length to act as dummy shocks, drilled so the ride height is what you want.
Then build the whole car up. When everything is installed weigh each corner, so you know what weight each spring is going to have to support. When you've got those weights you will need to subtract the weight of the wheel, tyre, brakes etc (all the unsprung weight).
I'll post how to calculate the actual spring required later.
Remember springs have TWO parameters, poundage and unfitted length. You need to know both to get the right springs
Bet you will get some replies that will give you a poundage to try, but not the length
