I mounted the tach more or less the same way. I took out the radio, and cut away quite a bit of the dash pad. It's not a very neat job, and I may redo much of this later. I'm going to mount a newer radio/tape deck below the dash, and mount a single speaker with it.
The dotted line is around a thin wedge glued to the back of the wood trim. This helps compensate for the curvature of the dashboard. This made a big difference in the fit of the whole assembly to the dash. Mounting this with hidden screws would be a real chore. I may change over to nicer black trim screws. The old radio hole, and much of the dash will get some more cherry trim. I need to add a nice clock, maybe an outside temp. gauge, and perhaps a cylinder head pyrometer.