Hmmm.. Its because I found a place in UK where they sell all sorts of different springs (mostly for rallying).. and I could pretty much choose what ever load rating I wanted if only I had the exact data on the standard ones.
The UK is loaded with companies like that... it's the worlds leading region for racing technology, period.
I think you need to talk to them about it. The diameter of the thread should tell them something about the stiffness of the spring, and if you are able to measure the weight load on each of the rear wheels, it should also be possible to specify the length of each spring, given the geometries of the rear suspension and the ideal ride height of the car.
The ride height is the critical parameter as the semi-trailing arm suspension is tuned by adjusting that, so in my opinion, you should always aim for the factory specified numbers here (unless you are specifically tuning the set-up for a race or a track day). Changing camber on the rear changes the road handling properties of the car, which you don't want to do unless you really know what you are doing. This is something the Matra engineers worked hard to get right, and with their racing experience and direct access to one of Europe's best test tracks, they surely knew exactly what they were doing.
- Anders