you need to use the sending unit that is calibrated specifically to your gauge.the factory one will not work.
search isspro's site for the proper sending unit.
with the factory gauge not worth much,there's really no point in worrying about it working.some guys want both,but ask yourself this;
if the good aftermarket gauge reads fine,but your stock one says your running hot,are you really going to pull over and let the engine cool so the factory gauge is happy even if you know the engine is still well under 240 degrees? if your answer is no,then why even have a dummy gauge wasting your eye sight time? lol.