What isn't said is the resistance values of the stock guage and the aftermarket one most likely aren't the same. If you try to splice into the stock wiring you can cause a misreading on the guage and a few other issues. As stated above, you need to use the sending unit that came with the unit or but one that will work with it.
yep what super six said so you would have to look that gauge up and purchase the corresponding sending unit and wire it accordingly with new wires. If you look down on the passenger side of your block you will see a brass block you need to tee off of that and add your sending unit.