Spring rates I think go by the weight of the car and not just 50lbs but, 100s or 1000s. Unless the weight in the front and/or back is significantly less than the 400E which, I don't think so. You should be fine. That is also why certain springs are stiffer than others. You can also custom make your set-up. Race stores have these at a cheap price also. I think Pegasus sells some Airplane manufacturer that produces springs. http://www.pegasus.com
Some other manufacturers including, H&R can customize a set-up but, of course you would probably pay a premium price.
One question I have for all you MB enthusiast. Why do you guys use H&R? I have used Eibachs on another vehicle and thought they were much better in durability. H&R springs usually cause trouble within time from personal experiences with my friends that have bought them. I am not talking about one or two but, like half a dozen people have told me their complaints. I remembered seeing applications for W-124 also for Eibachs. I could be wrong though on the application.