I found some soft aluminum, and made a strap ,and bent it to fit over the dash pad. It hooks under at the back then I pull it forward over the front edge and fold it underneath at the front. But before I did the final tuck at the front, I screwed a standoff to the strap,from the underside with flush fitting screws. And then screwed the tach-mount onto that from the top.
I formed the strap loose enough that I can slide it side to side on the pad. Usually, the tach is over to the passenger side a little ways, cuz I hardly ever look at it. When I went to the track, all two times,I slid the tach front and center, and I watched the tach hit 7000 and it stayed there at every shift,lol. I thought I smoked the clutch,but no,that wasn't it. One of the track officials came over and offered that I should lower my tire pressures, on account of the car seemed to still be hazing the tires at the end of the 1/8th. Nice guy, but IDK how he could see that far; 220 yards is a long way.
Also on the stand-off, I mounted two little 12vLEDS; a red one to indicate the line lock is on, and a Green one to indicate that the Overdrive is engaged.
I built that mount in 1971, and it has been with me ever since. It is about 1.5 inches wide, and maybe as thick a 3/16; I don't think it is 1/8th.
As an aside, the line-loc has two switches, a push-button momentary on, mounted to the stick as usual. But I also installed a toggle on it to make it a Hill-holder at redlights.
Oh-oh, I gave it away; my car is not an automatic, and not a column shift lol. The tach don't care,lol.
