Cutting springs is not safe.

However, if you can find the w123 shop manual online, you need to find page 23-010 Permissible combinations springs -- shock absorbers, or you can post the extension of your chassis number (123.133 is my '85 300D, for instance), and I can provide you with the correct part numbers.

Have you considered that your rear suspension might be sitting a bit low? I was at a show Saturday with a 300d turbo with 55,000 miles on it, and noticed that the rear seemed low - the rear fenders have a different arch to them than the fronts. My 300D also sits lower in the rear than I think it should, I plan to swap out the rear shocks shortly with a matching pair to the fronts to see if that helps. (well, and mostly to improve the ride quality a bit more)