With the original current-pulse tachometer, some of them, mine included, the white wire is not in two pieces joined at the tacho, it is one piece which is threaded in a loop around the metal of a removable loop on the back of the meter.
So all you would need to do on that type is detatch the outer U shaped piece of metal that is the outer piece, free the white wire and put the U piece back on the tacho. Then remove and insulate the supply voltage lead and remove and insulate the light bulbs in their holders.
Ensure that the black earth lead doesn't touch anything live and if the vibrating voltage regulator (for the fuel gauge) was mounted on one of the tachometer fixing bolts, put it on to one of the other instruments instead.