Greg
In your '49 GMC the temperature sensor was probably an electric one mounted in the thermostat housing as shown in this photo. '49 temp sensor In my '52 the temperature sensor is a mechanical bulb that is mounted in the cylinder head. It is located behind the carburetor in this photo. '52 temp sensor The bulb screws into a bushing that is screwed into the head. Most of the vendors carry this bushing if you don't have one.