On my 87 blazer I put one to the right of the instrument cluster on the surround. That part is more easily replaceable and you dont have to put holes in your dash. It also keeps it out of the way of the windshield but still very easy to see.
mine is on my steering collum cover. infront of my dash, but my dash is different, cuz its a saudi. if you mounted it there you would block some of your guages.. they have tachs that can fit in a pillar.
why would you put a tach on a auto? it is slower if you manually shift it? unless you are runnning 6krpm you dont need it, i am running 6200rpm and dont use on? kind of like putting a boost gauge in a car without force induction.