The rear end - when stock - isn't adjustable, however, fastlane used to sell eccentric trailing arm bushings. I don't know if pelican still have them...
...you'll have to look...
If you want to see the springs fitted to W123s in a nice long list then look through chapter 32-250 in the FSM (don't just restrict yourself to the W123 FSM - the same chapter number is used on other models - however you probably won't be getting anything other than W114/115 W116 W123 or W126 springs to fit)
If you want to calculate spring stiffness / rate then have a look here
More than you are likely to ever want to know about OM61X piston vacuum pumps (sure I'm droning on about vacuum pumps in this thread but there's coil spring calculations buried in there too)
If you need help making calculations just say.
I don't know exactly what you are trying to achieve however but there was a guy in England doing something similar - I can't remember his user name but he did post here as well as on BW asking similar questions