What's the best way to fix aluminum intake threaded hole for temp gauge sending unit?

The end of the sensor must be in the flowing hot water, in order for the gauge to read right.
If you install that temp sender into a stand-off pipe, the water in the pipe will be cooled by the surface-area of the pipe, and read cooler than the water passing by at the bottom of the pipe. and so, your gauge will read erroneously, all the time.