needs to be a diesel tach... a gasser tach wont work. gasser tach is looking for a signal from the coil... sine wave vs. some whozamawhutzis and all that.
to be honest i havent looked for a W-terminal, dunno what it looks like to describe it... but i believe it should have either a W or a number stamped next to it. a "25" maybe?
if you have your alternator cracked open at a rebuild shop of some kind, they can usually tap into the signal you need and give you a terminal to hook up to... even if your alternator didn't originally come with one.