Sending Unit for Temperature Gauge

The engine temperature gauge in my 76 Dart Sport with a 318 never gets above about 120 or 130 degrees, even in hot weather. Replaced 180 thermostat with a new 180 thermostat and gauge is still reading too low. Next thing I want to try is replacing the sending unit with a Standard Motor Products TS17 sending unit. Do you think this will solve problem? How much coolant will come out when the old sending unit is taken out?