Gauges are an important part of completing any swap. In most cases the process is pretty simple, but there are many options and different ways to solve gauge problems. I'll attempt to describe my own solutions and highlight other options where appropriate. 

Here are the gauges in a 380SL as well as the applicable idiot/warning lights

Speedometer

The speedometer/odometer will either be mechanical or electronic.

Mechanical: If it is mechanical, there will be a cable that connects the transmission with the speedometer module in the cluster.  Hooking up the speedometer in this case is just a matter of connecting the two components. If there is a significant difference in the rear end ratio there could be some variation in real vs indicated speed. In this case it is possible to change the gears in the speedometer unit that will correct for this.The 300D has a mechanical speedometer.

Electronic: Electronic speedometers also get their speed reading from the transmission, but the signal is actually a square wave signal which is generated by a VR (variable reluctor) sensor and a trigger wheel inside the transmission. The 380SL has an electronic speedometer

Discussion: Because of the discrepancy between the two types, a choice needs to be made. The simplest solution is to keep the mechanical setup on the transmission and get a manual speedometer module from the junk yard and swap it into the cluster. The alternative, and better solution, is to keep the SL's cluster with the electronic speedometer module and adapt the VR sensor to a trigger wheel inside the transmission or on the driveshaft. There are numerous different combinations that are possible so the particular solution for any individual case will be dependent on what might be available

An Electronic Speedometer Solution

Tachometer

The tachometer is another situation where there are several different choices.The optimal solution will depend on where the signal is being generated. There are two possibilities

Vibration Damper-the earlier OM617 engines got a tach signal from the vibration damper on the front of the engine. There is a pin and a sensor that generates one pulse per revolution. This signal is passed to the tach amplifier, which is generally on the drivers side of the engine compartment. This amplifier in turn passes a signal to the tach in the gauge cluster which reads RPM. If you have this setup, you can just swap out the tech module in the cluster and hook up the wires as in the donor car. 

Flywheel - the later OM617 engines ('84-'85) took the tach signal off the flywheel at the back of the engine. This signal was fed to the EGR Controller which in turn passed a signal to the tachometer in the cluster.  If you have this setup, then you need to use the tach module from the donor car and swap it into the cluster. Note: you will also need to use the gauge face because the tach sweep is different. The best way to do this is to glue the face of the 300D to the face of the 380SL.

Oil Pressure

 Oil pressure can also be mechanical or electrical.

Mechanical - if you choose to go mechanical, you will need to swap a mechanical module to the cluster and then run a oil line from the oil filter housing to the gauge. The 300D comes with a mechanical gauge.

Electrical - the 380SL has an electrical oil pressure gauge. The simplest solution is to take the sender from the 380SL and screw it into the oil filter housing and connect the wire to the gauge in the chassis.

Coolant Temperature

Both cars use an electronic temperature gauge, so just hook up the SL gauge to the 617 sender and you're good to go. 

Fuel

The fuel gauge and sender are completely compatible with diesel fuel, so no change is needed to make the fuel gauge function properly.

Grounds

Gauges require good grounds and improper grounding is one of the chief causes of problems and issues with the cluster. It is highly recommended to run an additional dedicated ground from the cluster to the chassis. The internal ground can fail and result in weird and difficult to diagnose problems. To make it easy to take the cluster out, put a quick disconnect connector in the line so you can easily unhook the cluster for maintenance and troubleshooting.

Cluster

The Mercedes clusters are all modular and many components of different clusters can be used in other clusters, even from different classes of vehicles. Be very careful though, as gauges that look alike, mount alike and have the same function could be completely different.

While you've got everything torn apart, it would be a good time to replace all the bulbs and check everything out so you don't have to try to get at it at a later time.

The bulb to the O2 sensor warning light should be removed, because there is no O2 sensor being used.