it's pretty simple to swap over actually,there are two extra brake lines to the master cylinder and ABS pump and you need the loom to connect to the module (ECU) from the pump and to the power,including the OVP relay ( silver box with red fuse holder on top. You need the W126 front disc rotors with the pick ups for the ABS. there are two sets of wires on the front discs,one is the pad sensor the other the ABS.
it all unplugs so it comes off separately. The W126 Rotors fit straight onto the W116.
There is a single line from the ABS module running through the car to a dual terminal block under the rear seat. remove the cable and disconnect it without bust ing it. Now the hard part. the rear wheel sensor is in the diff housing so you will need to swap diff heads,or if you want to keep the same ratio ,swap the housing so you can retain the sensor.
The cable locations speak for them selves.
The easiest way to hook up the ABS in a W116 is to have the donor W126 beside you so you can disconnect the brake lines etc and fit them to the victim... The bracket for the ABS must be welded into the W116 to secure the pump .
All in,it's about 2-3 days work from go to whoa . ABS is great in wet or slippery conditions allowing the driver to brake hard with out the car going all over the place.