Hey waggin -
That's exactly how I mounted my tach in my original '69 Dart waaaay back then. Unfortunately the local police felt it 'impaired my vision' and gave me a "fix-it" ticket.
I spliced in a connector to the wiring, put the tach in the glove box, got the "fix-it" ticket signed off, drove out of the police dept. parking lot, plugged the tach back in and put it back up on the mount.
Never got bothered about it again and never saw another mounted like that until the tuner-boys started having to show off all of their look-at-me stuff in the past couple of years.
Gotta agree, it was extremely easy to see there.