As others have already indicated, you have several options. By far, the simplest choice is to install a mini tach in the space for the clock, wire it up like a modern tachometer and you are good to go.
If, you want a "correct" installation for concourse purposes, you will need both a 6000 rpm tachometer and the 120 mph speedometer with resettable trip odometer. The main underdash wiring and the cluster harness will need to be modified to allow for the tachometer and the switch from Ammeter and Oil Pressure gauges to Oil and Alt Charge warning lights. The charging system wiring under the hood also needs some modifications and the oil pressure sending unit needs to be replaced with one for a warning light. If you are comfortable working with automotive wiring or are planning to replace the main wiring harnesses anyway, this should pose no problem. Otherwise this might be a challenge.
Last, but not least, remember that these tachs are 46 years old and may need refurbishing to function. There are vendors that do specialize in restoring original functionality. There are other vendors that will convert the factory tach with modern electronics that require slightly different wiring.
At the end of the day, the differing cost of these options will usually drive or influence your choice.
Good luck with your choice.