The hood tach on a Firebird with a 400 hood is awesome. If it's not in your PHS docs I don't think it will hurt the value, IMO. Most Tachs were dealer installed options. I'm cutting a hole and installing a 67 hood tach because I like that one over the 68. Both were available at the dealer so it's very possible the tach could have been purchased at dealer, installed and the customer received his car.

I believe in NOT changing the car from it's original configuration when making it a show car but some things are debatable. Some people have changed from Points to HEI, something I won't do as it is a obvious visual change that does not add visual value. To each there own ...