I have a factory tach cluster in my '75, and the tach is a separate 3 wire plug (power, ground, signal). I does not use the circuit board or plug for the main cluster.
You need to get a wiring diagram of the large cluster plug and determine if the pinout is different from one year to the next. If they are the same, it just plugs in and everything works. However, its possible that one or two wires changed over the years. That means you may need to swap/splice some wires in your truck so that they match the pinout of the new cluster.
Bruce