W123 rear spring upgrades

[mark82]

New 240d, been lurking, time for some upgrades.

I would like to improve the rear springs, less mush, less lean, better loaded ride height. Dont want to lower it, in fact maybe 1/2" higher would help, dont want a skateboard ride. Mostly want to tighten things up 25%.

Located one supplier that has progressives, but they sound like they also lower. Another that has upgrade kits for the sls and stock suspensions. Neither provides any specs for me to evaluate whats going on.

Does anyone know the factory spring rate and free length?

Does anyone know what the factory HD springs (if avail) change?

Does anyone have source recommendations?

Thanks