Typical fuse box has a unused slot at 1 end. The fuse clip for hot at switch on is there but the clip for line out is missing. The factory tach wiring included that fuse clip to insert there. If you dont have that factory wiring, a inline fuse holder stashed behind the column door works. Other end of this wire goes on the longer of the 2 contact studs on the tach.
Signal wire from coil goes on the shorter of the 2 studs.
More important is the chassis ground. The tach is grounded to the inst' panel via the 3 mounting screws. Then the entire inst' panel relies on another hardware link where it mounted into the dash with screws.
Add a real ground wire. Attach it with a ring terminal at the lower center of the 3 tach mounting screws and route it to the chassis behind the left kick panel. Sheet metal screw, ring terminal, and toothed washer required. Basically, copy what factory used at ALT gauge connections everywhere you can. A male/female spade terminal disconnect about 4 inches below the tach ( also stashed behind column door ) will come in handy too.
Also be aware that the circuit boards inside these OEM tachs don't live very long, especially when attached to aftermarket / high energy ignition systems. Aftermarket offers a newer more compatible circuit board too.