The big question is are you putting this car together for you or the next owner? If you are worried about impacts to future worth, then you are doing the latter when it comes to adding additional useful and tasteful factory options. If you look at those items as things that your car would have come with if you had the chance to order it when it was new, then you are doing the former.

Being a car guy, I always want full instrumentation in my vehicles anyway , so the correct Pontiac hood tach is not just a styling cue. And I want it correct so no just clamping a Sun tach on the steering column.

Yes, the scary part is cutting the hole. Be very careful of the measurements and question the instructions that come with your tach. Mine said to get a hole saw so large it would have cut out the 3 spots where the tach bolts to the hood. I knew that wasn't correct. And if your hood is painted, use lots of painters tape.