It was initially a dealer installed option in 1967 on the GTO. Not sure if it was available on the GP so you may have to place it on your own. These are the GTO instructions.
If you measure out 9 inches from the rear of the hood (cowl), then measure out 10 inches from the edge of the hood (driver's side, at fender), the intersection of these 2 points will be the center of where to drill a 3-3/4 inch diameter hole. Next. set the tach into the hole and mark the location for the 2 stud holes. The tach should sit into the holes with the front edge touching the hood 4-5/8 inches from the hood edge (cowl)....According to my calculations this WILL involve cutting into the brace under the hood skin. This will make more work, but a better job as the tach will have more support........I haven't installed mine yet, as I have the original hood on my car....there isn't enough JIM BEAM inthe house right now to give me the b-lls to take a 3-3/4" hole saw to the hood I hope this helps......put masking tape on the hood first and lay the job out......Measure 25 times then cut