The sender is nothing more than a variable resistor. Take it in your hand (disconnect truck wiring first) and hook the electrical post to an ohmmeter and the other ohmmeter lead to the ground or base of the sender. Turn the ohm selector switch until you get needle deflection or a reading on the digital meter. Heat the part of the sender that goes into the block with a match and the meter will start changing values. You can check the gauge by replacing the sender with a variable resistor once you figure out what wire go where. Now the cold hard facts. The sender and gauge are matched up resistance wise. If you don't match them you will get a reading on your temp gauge, but it won't be right. So why bother. Same thing goes for the gas gauge. This forum is full of questions from people asking why their gas or temp gauge don't read correctly.

Six volt guy living in a twelve volt world