ok, went and took a look at my convertible. Originally it had standard guages. I converted to stock Tach and center guages.
Again, it was 25 years ago. My tach runs through the firewall just under the passenger side brace. This makes sense, the tach is wired into the coil and the coil was on the passenger side. It's a pretty fat red wire and there is a grommit that looks like it was part of the wire. I had pulled the tach out of my wrecked Mach 1, back then I would have been careful to put it in the same spot. I don't remember drilling a hole for it. Check to see if you have a small plug there. Perhaps 12mm in diameter.
