This is the description of the BW WT358 sensor you have listed:
Quote:
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Temperature Sender With Gauge; In design, BWD specifies custom wax formulations that will expand when heated, activate a snap action electrical contact and transfer the electrical signal to the fan controller. In manufacture, BWD custom form the wax pellets for different temperature range specifications. All units are calibrated & tested to confirm switch actuation at their specified temperature. The switch is designed using a formed bimetallic contact arm. The bimetal contact movement is achieved using the different expansion rates of two metals fused together. This is a creep action device, provides precise performance. In production, all thermal switches are factory-calibrated to make electrical contact at the specified lower-temperature set point. BWD switches feature a robust design with high grade materials to meet the stringent demands of today's service professional. BWD is a global manufacturer of original equipment ignition products, complete quality control is maintained through the manufacturing process from componentry to finished product.
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Sounds to me like that specific sensor is intended to control a fan circuit by the way their description is worded.
The 213-4792 AC Delco sensor is coming up as a gauge sensor, but it also shows that fitment is 1985+ Chevys, which may not be compatible with your gauge, and thus why you might not be getting a reading from it.
The AC Delco 8993146 appears to be correct for 1979 and later gauges, and I do realize that you have a 1979, but that was also a crossover year for gauges and you may have late 78 gauges and wiring in your truck. You may need sensor 1513321 if this is the case.