You aren't running a new wire. You are simply disconnecting the wire that already goes to the (+) terminal of the coil at the ignition switch and installing the tach in series on that wire. I presume the banana plugs should be sexed correctly on the Rotunda. You will need to take the ignition switch out if you haven't already.
Speaking for a 62, the wire you are looking for is actually red/green stripe. You attach the red wire here. That is 12V for the coil. To verify that it is the wire you want, it will only have 12V on it when in the "Run" or "Start" position and not in "ACC".
The black wire on the tach should connect to a wire that looks like this, which is my 62's fried resistance wire. It may be different for your 61. It already goes to the (+) terminal of the coil. This wire actually has "Resistance" printed on it and is pink in color. You will not see this wire color at the coil as there is another circuit involved that bypasses the resistance wire at startup to provide a full 12V to the coil when cranking.
This may be a bit confusing, but this is the circuit schematically. I have found that it is not physically wired this way. This indicates that the pink wire goes directly to the coil. The brown wire from the starter relay is what goes to the coil.
I also repositioned it so I can just glance down to see my RPM's. it works just fine 
