The sensors are different:
* Gauge sender is a simple variable-resistance type (1-wire)
* Temperature sensor for fuel system is a precision voltage type (2 or 3 wire)

You want to measure water temp at the engine because it's a regulated temperature, while the temp in the radiator and radiator hoses is unregulated and can vary wildly.

Nissan Water Temp Sensor, bolts to the cylinder head or coolant passage of intake manifold



Water Temp sensor on factory A14 EFI manifold (B)



ECUs use different temp sensors: Some use water temp sensor, other use intake air temp sensor (IAT), some use both. Datsun EFI had an air temp sensor in the airflow meter, while the ECU read the water temp sensor. Modern mass airflow sensor doesn't need to know air temp. So what you need all depends. Modern ECUs like MegaSquirt and Haltech can use both temp sensors (water and IAT). They can both use either Nissan or GM sensors.

With a DIY ECU, most people use a GM Intake Air Temp sensor, just poke a hole in the air duct and insert the IAT. Nissan IAT sensors bolt into the inlet.