Tach on an old engine

[Fayetteguy]

So i was wondering if anyone has any experience of adding a tachometer to their boat. Ive got an old Mariner 40 which it appears tachs are made for and seem fairly easy to install. Just looking for some insight. I have decided that i may need to swap props as my 16ft cs with a 40 is at 27mph WOT and it seems as that it should be going a tad faster. I know speed isnt a big deal but i would like for the boat to be set up for optimum performance and i need a tac so i can find out what kind of RPM's this ting is turning.

-matt

(hopefully its as simple as a Small Block Chevy ;D)