mechanical temp gauge location

You can buy (but you'll have to search) thermostat housing spacers that have additional tappings. "If you were careful" and if the sender (such as EFI or the original 1/8" NPT Mopar) you could likely tap it into the water port where the intake "blanks" that port at the rear of the block.
Bear in mind if you put it above the t stat and or the higher you get it---that in the event of a stuck closed stat or a leak that exposes the sender---it may not respond.