I would use the original springs if you have them, unless they are visably bent in the previous event. (I have never seen the spring hurt in any of the ones we have repaired).
The proper springs for a 380SL #115 321 29 04 aren't all that expensive at $99 list (each). Replacing them and the spring shims will be the real answer. The shims 107 321 02 84, 03 84, 04 84, and 05 84 come in increments of 8, 13, 18 and 23mm respectively. I would probably recommend the 13mm for first try since your have old rear springs (I presume). If the original rubber shims are whats on the car I would look at the ridges ("burls") as they denote the size 1 = 8, 2 = 13, etc. Use one size less to allow the new front springs to compare with old rear springs.
Changing shims is not easy so you don't want to do it many times, but thats about the only way to achieve a desired height. Alignment will have to be corrected after each change.
Bosch Master, ASE Master, L1
33 years MB technician