Forces seen by the spring are not the same as the tire contact patch as there is leverage involved.

Are you looking for height alone or height + stiffer spring?

If just height, measure spring length at stock ride height. ( Or measure between the lower control arm and body _inline_ with the spring ) Jack up the body until you get the desired ride height and pull the same measurement again. The change in height is the thickness of the spacer you need to make.

This spacer can be fixed, a sleeve with washers you stack to adjust height or a threaded spacer ( AKA "hidden adjuster" used by oval track cars ) .

If you want stiffness, go directly to aftermarket weight jacks and universal aftermarket springs as this will be less fussy than chasing stock stuff and possible less expensive as there are many used springs out there.

See my posts in the thread below, ignore what the original poster has to say as he was insistent on ignoring my experience.

http://www.peachparts.com/shopforum/performance-paddock/342744-w123-racing-setup-serious-questions-within.html