Looking at a 67 Coronet tomorrow, advice.

[YY1]

Several very similar styles.

I was thinking- strap or conduit clamp mount to the rear view mirror base, or sit on package tray.

 

[Dave6T4]

The one I had just double-sided-taped to the top of the radio under the dash. Totally invisible.

 

[Gary Stavely]

Do you think passenger side outside rear view mirror is original to the car, or later reproduction? Station wagons usually came with them, but sedans and hardtops, not so much. It could have been dealer installed, like the radio.
Classic Industries in Los Angeles sells an aftermarket tach that fits in the unused clock hole in 1966-67 Coronet dash. Your car also came with fender top turn signal indicators usually seen only on Coronet 500's. I don't know if they were an option, or dealer installed.

My 440 has the fender mounted turn signal lights, I do believe they were part of the high trim package. Standard on the white hat specials I believe. That car is not one of those, so probably a option...