I have replaced some in a friends 500SL, his SRS system spontaneously went off. Before you unplug or plug in the SRS, make sure you disconnect the battery. Other than that their isn't much to replacing the seat belts. Have you already bought new belts, if not I have some out of my 87 16V for sale, not sure if they are the same but I would think they are.
MBCA, PCA, BMWCCA & SCCA* member
86 190 2.3-16
87 190 2.3-16