With the speedometer cable disconnected you will be able to pull the cluster out further for more working room.
Starting on the right side, begin disconnecting the items that attach to it.
First, you will pull out any light bulbs that plug in to the right hand side of the cluster. On a diesel W123 this will include the glow plug bulb and seatbelt bulb. Then unplug the spade connector for the power supply to the clock.