The FSM says to jack the engine up on the oil pan or use a engine hoist. Trans mount is done all by its self. heres a little write up on how I did it on my 82 300sd w126. Russ H.
Read this First http://mb.braingears.com/123_DISK2/program/Engine/617/22-211.pdf This site has a DIY with pics for a 300SD W126. I looked at it when the site was free, but now he wants $30 year subscription. It wasn't worth it to me back when he started charging, But I looked at the site today to see if it was still there and he has added alot more. http://www.pindelski.com/ Check your oilcooler lines, if they are leaking NOW is the time to replace them while your Motor Mount is out. 1) Raise front of car (I used ramps) 2) Remove air intake hose between aircleaner and radiator support 3) Unclip fan shroud from radiator, pull shroud up so the tabs on the bottom clear the radiator then hang it on the fan as close to the engine as possible and away from the radiator. I don't know if you can get the shroud all the way out on a w123 or not. The reason for this is you don't want the fan hitting the shroud when you jack up the engine! 4)Remove throttle linkage shaft. Look at this pdf. Page 2 is for your w123. The picture is bad but there is a diagram with # 's and what to remove and how to do it on the left. http://tinyurl.com/eygy2 You need to do this so you don't break linkage parts when you jackup the engine. 5)Crawl under the car and remove the nuts,washers,rubber bushings on the bottom of the 2 engine shocks. You will most likely need an oddball 7mm open-end wrench to hold the flats on the shock rod.There is a pic of the flats in the first pdf.(adjustable wrenches are not thin enough, and don't even tell me you are going to use vise- grips) 6) Start on the passenger side and look up thru the hole in the body below the motor mount. you might need a flashlight. Use a 8mm allen socket and long extension to remove the center bolt.( I made sure there was not any dirt or grease in the head of the bolt. didn't want to strip that one) Then remove the 2 bolt's that hold the mount to the body. (I think they are 6mm allen) The one closest to the firewall was easier for me from under the car. The other one from the top. 7) With the drivers side mount still bolted down. Get your floor jack and a block of wood (I used a 6x6 a little over a foot long) put it between the oilpan and jack. I didn't like the Idea of jacking up the engine this way,but the oilpan must be extremely tuff it didn't even leave a mark on the pan.(If I had a engine hoist I would have used it instead) Now slowly,carefully jackup the engine until you can remove the mount. Now try the new mount, you will need to jackup the engine higher to get it in place. Replace the 2 outside bolts ( I put anti-seize on these, I plan on keeping my car long enough to do this again) Lower the engine carefully(make sure the engine shock goes back in the right spot) back down on the mount. Remove the jack from under the car. 8)Crawl under the car with a flashlight and look up where the center bolt goes. Are threads the centerbolt screws into 100% lined up with the hole in the mount?(the pindelski site had good pics of this) DO NOT ATTEMPT TO START THE CENTER SCREW UNLESS THIS IS 100% LINED UP!! If you strip or damage the threads in that housing you are in for more work and $. Don't stick a screwdriver up in there either to try and line up things, those threads are only aluminum. Now the hard part you need to pry, push, horse the engine the way it needs to go to line things up. Make sure what ever you pry, push,horse on is substantial and can take it! A small porta-power with a duckbill spreader and a light touch seemed to work for me. It took me a good half-hour of moving and checking before things lined up. When it is lined up and only then put some blue loctite on the centerbolt and torque it to spec. 9)Now do the same for the drivers side. The only difference is I had to use a regular L-shaped allen wrench for the 2 outside bolts, and have the engine jacked up to reach them. Be safe and put a small block of wood between the mount and engine arm housing. Also the drivers side on my car has a tin shield that fits between the mount and the engine arm housing. With the shield in the mix it took me closer to an hour to get things lined up. 10) Now you can put everything back together. You will notice there are little grooves in your fan shroud and radiator at the top where the clips are. That's so you can raise or lower the shroud. Make sure you have even spacing all the way around fan.If I remember correctly you can slide the shroud sideways too. Enjoy your smooth idle! Russ H.