Can a after market tach be put on a 240sx?

yes, check out autometer.com and they have info on what color wire to tap into. I assume you have a tach, so what you do in run the tach pick up wire to the distributer (where all the spark plugs are) and there is a couple of wires going in the distributor, one of those is the one you need to carefully tap into. It will vary based on year and model.