[OT] Adding a tach to a Hondamatic

[doubt at inwave.com (Nate Mellom)]

All,
I have a '79 Honda CM400A Hondamatic my uncle gave me a while back.  The 
only gauge is the speedo, where the tach normally resides on a manual 
bike lives the gear indicator light.  I'd like to add a tach (something 
small, like a 2"), but I'm not exactly sure how to do it.  Do I need a 2 
cylinder tach, or would a generic work? I'm sure I can figure out the 
coil connections, I haven't looked in to it too much yet.
Any tips?
Nate