If you are going the coilover route, I would use a piece of 1"x1" square tube and drill holes for the distance of the shock at ride height, and bolt them in.
Keep in mind that a 12", 14" shock etc., will not necessarily ride at that height. I usually go about 3/8" below the advertised measurement.
After bolting in the 1"x1" in place of the shocks, I weight the car. (Front and rear).
Also, keep in mind that the suspension, rearend, wheels, tires, etc., are unsprung weight. The springs do not hold these items up.
So you need to subtract the weight of these items from your front and rear totals.
The angle the shocks are mounted on will also require a different spring rate.
Give all this info. to the spring supplier, and they can make a more informed selection. Don't order springs from a parts warehouse, they only know what the "book" says. Go straight to QA1, Eaton Spring, etc.
I would rather go through all this work and have the right spring rate. You will be more satisfied with the "ride" of the spring.
To stiff and to soft stinks, just ask Goldie locks......