How to Install a trans temp gauge.?

Typically you don't run a trans temp gauge with a manual.

There is no bung to install it, and there isn't a spare port anywhere either.

You could install it in place of the drain plug, but on the 3650 it's on the bottom and chances are first speedbump you drive over would rip the sensor out...plus it may interfere with the countergear inside.

Other option is custom weld and tap a bung on the trans. This would involve pulling and gutting the trans to do, so not a good option.

Overheating isn't really a problem with manuals unless you run low on fluid, or have it built with overly tight tolerances.