The tach was an option on 85/86 diesels. If you have one of those years, the wiring should already be there. You just need the tach, the circuit film on the back of the cluster that has the tach terminals (same as gas circuit film) and the sensor that screws into the injection pump gear housing. 85/86 will have a threaded plug there if no sensor is installed. 83/84 diesels didn’t have the tach option, and the gear cover does not have the port. It is a good bit more work because you need to swap the gear cover from a later 6.9 or 7.3 IDI. You really have to know what you are doing so you don’t get the engine out of time. Then you need to add the wiring yourself. I have a diagram if you need it.
SHORT BED 4-DOOR DIESEL: 1986 F350 4x4 under construction-- 7.3 IDIT ZF5+GVOD
STRAIGHT SIX 4X4: 1981 F150 2wd to 4x4-- 300 I6 close ratio diesel T19, hydroboost brakes, Saginaw steering
BIG F: 1995 F-Superduty under construction— converting to 6.9L IDI diesel ZF5+DNE2