do you have to take the complete dash out to fit the taco or can you just take out the instrument panel

hmmmmm

I unmounted the dash and pulled it out about 300mm, just enough to undo the clock retaining screws and the mount the tacho.
You'll need to disconnect some plugs at the top RH corner to get enough slack, and I also disconnected all the heater control cables, choke cable and removed the glove box lid and box.
Pain in the atrium
Then I found I had no wiring for the Tacho. so I had to run in a pair of wires to the ballast resistor in the engine bay.
Of course, there are 2 ways to connect those wires, my first choice was incorrect.
If the Tacho. doesn't read with the engine running try again.
Easy enough to remedy, just reverse the connections at the ballast resistor.
Don't forget the dash lamps need to be shifted from the clock to the Tacho. and you need a battery connection and an earth connection, all covered in the Technical Section on this site.

thanks for that ,much appreciated

red to pos green to coil and black to neg bud

Aftermarket tachometers are wired that way.
Nissan tachometers are wired in a totally different way.

See Wiki - Tachometer Wiring