I just did a drum to disc changeover using a rear axle from a 250SL in my 230SL. It is a big job contrary to some opinions. You need a new master cylinder, a rear brake proportioning valve and mount; This needs to have holes drilled into the trunk floor and be mounted above the rear axle and is very expensive, all brake lines and the hose from the proportioning valve to the calipers; depending on the condition of the donor axle, but the hose should be replaced in any case, you may need calipers rotors and brake pads, again depending on the axle condition, I would replace the pinion input seal and check the pinion preload, this requires an inch pound dial reading torque wrench [about $100.00 used on ebay] a clicker type wrench wont work, and a very special socket for removing the pinion nut, some here have had success making one but my axle was on so tight the homemade wrench didn't work, and even with the $90.00 socket I had to put my Snap On puller on to the pinion flange and position the puller over my socket wrench to hold it onto the nut as I tightened it as you cannot use an impact to tighten it to get the proper pinion pre load. All the rubber pieces should be replaced and the the split boot also replaced. It is a real big job according to Doctor Dan to separate the axle tube to put on the one piece boot so I used the split boot with success. It is a good time to replace the axle output seals and axle bearings also.
You can just bolt the axle up in there and use all the old stuff, but most of these are 40 plus years old and all the rubber stuff is shot. And you absolutely must have the proportioning valve and master cylinder. I also completely cleaned and painted the axle, many many hours just doing that. This turned into a major job and even having the proportioning valve from the donor car it was very expensive also. Just the special tools cost quite a bit. Again you can just put in the axle and hope for the best. Remember to fill the axle with gear oil while the car is level on the ground with the wieght of the car on the tires as you can overfill it with the axles drooping. Also be very careful not to let the axle tubes swing down hard as you can crack the casing where they hit, very easy to ruin your axle that way.
You also need to make an alignment tool to center the axle on the final installation. Also be sure to unhook the rod that goes from the axle to the frame on the right side, these get bent often by dropping the axle with it hooked up, this rod also has four rubber mounts, two on each end even though parts blowups show that the mount to the frame only has one big donut, it has two, one hidden in the frame. That rod centers the axle by moving the rod, make sure that is all freed up and moves easily so when you get it back together you can loosen the lock nuts and turn the rod from the end with a straight slot screwdriver to move the axle side to side. There is way more to the job, do some searches here.
