Want to install a tachometer in my 81.

Currently got:
Temp light
Oil Light
Volt light
Big center speedo
Big center fuel gauge

I seen some 70-78 tachs for sale online, but i dont know if i need to hunt for something special to make this all work. was told its factory pre-wired for tachometer. What to do? Source parts car/junk yard or easier to spend the big bucks and order one?

thanks!