Sensor location???

[CEE1NG_RED]

I have a '78 350 motor with a sensor coming from intake manifold and driver side head cylinder that are defective. Now, what is the size of those sensors? What are its purpose? My temp gauge don't work and I want to monitor my motor as I am flushing the coolant. Does the temp gauge connect in the driver side head? There is no sensor on passenger side. I want to get the best reading and don't know if the temp gauge is suppose to go on the passenger, driver heads or intake manifold? Is Sunpro a good brand or should I go Autometer? Also my truck has no fuse box wiring so will I still be able to install a temp gauge and any other gauges as well?? Lastly when taking out these faulty sensors do I need to drain the fluids or no? Please help. Thank you
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