[dmsfrr]

Is the truck converted to 12 volts? Does it have an original working water temperature gauge?
Are there two sensors on the flathead engine connected to the Temp gauge?

One of the Temp sensors from the flathead will need to swap onto the engine from the Fairlane because it electrically matches the gauge in the truck. The '56 engine only has a threaded insert for one sensor, drivers side head near the rear.
The existing '56 Temp sensor in the Fairlane engine will not play well with the original Temp gauge in the '52 truck.
Unless it's really broken do save the '56 Temp sensor, they are a one-year-only part, hard to find and pricey if you need to replace it.

If I'm reading the attached diagram correctly, the sensor from the right side of the flathead would be the one to use on the '56 engine.