I have worked on cars for nearly 50 years. Although I have never worked on a Mercedes (even though I have a 450 slc) I would say that you are safe to place jack stands on any portion of the rear suspension that is clearly well built, does have a brake line or rubber boot in the way, and that is as far outboard (more stability) as possible. My MB has jack points on the rocker panels which tells me that they are designed to carry the load of that portion of the car. I would check these panels for rust before I lowered the weight the car on them. If you are using a roll a round type floor jack be careful when you jack up back of the car after you have already put stands under the front. Floor jacks jack in an arc and they can pull the car off the front stands if the jack does not roll under the car as you are jacking it up. What ever you do, DO NOT scrimp on the money when you buy jack stands. Buy good quality. DO NOT buy the ones that are made from a pipe that is split into 3 legs. Wal-Mart has good, 4 leg ones for a reasonable price. I weld approx. 12 x 12 x 1/8in. plate on the bottom of my stands because I often use them on the gravel driveway.
While you have the engine out you should install a nice little 4 spd. manual trans. that I have for sale. It is out of a 350 slc and I am pretty sure would bolt up to your 380.
Good luck. Kip.