Trans temp gauge sending unit
 

Was wondering on some input on the best location to mount the sending unit for my mechanical trans temp gauge for a power glide. Thanks for any info.

In the oil pan where the oil is!
 

In the side of the oil pan in a location where there is no interference from the valve body. Usually the rear passenger side is the best place to check with the pan off. Cooling Line temp is not what the trans clutches and torque converter see, that is only a lube circuit and not what the oil temp is that is circulating in the transmission.
Bruce