A few factors here.
1. The "Tach" dashes have a different metal back, which allows for the mounting of the tach.
2. The gauge layout is different, and you will have to re-mount the missing gauges in other locations.
3. The wiring is different under the dash & hood, as the factory Tach dashes use an "ALT" light, and the standard dashes have a amp gauge. Different circuits. The factory tach cars also have an oil light, not a guage. In addition, the wiring for the tach from the ignition switch to the tach to the resistor wire to the distributor also needs to be accounted for. Odd as it seems, the Cougar harnesses will work in this application.
Hopefully some of the members here cab provide wiring instructions to wire in the tach, but be aware of the other aspects of this swap.
(PS - I did this in my 68 back in 1981, but have since lost/forgotten the info. I do remember changing both the oil & temp to mechanical gauges, and moving the gas guage to the top-center / clock location.)