Provided its installed correctly and looks like a factory install...rather than something some teenage kid glued to the hood....
too the purest I'm sure it will be a point that sticks out as a glaring "non-stock addition"
to probably 99.7% of the people looking at the car, it will be a rather interesting additional option,
and Id bet if you simply tell them, it was a rather rare option that year, they will believe you.
keep in mind even most "KNOW IT ALL" enthusiast would have to check reference material ,too feel sure enough to dispute ,what looks like either a rare option, or a dealer installed option. and I,m fairly sure that was done on dozens if not hundreds of cars back then as most if not all, dealerships would readily do minor custom work on time and materials basis.
back in the late 1960s it was far easier to simply go to the dealership parts counter, buy an optional piece of equipment and walk over to the order write desk and slip the guy $20 and some mechanic $50 to have some minor custom work like that done at most dealerships on time and materials, billing, in fact I know several guys that had non-factory custom work done.
as an example
I know several people that had the dealerships add a kit that was sold in the early 1970 that allowed the cosmetic hood scoops like on the 1967 formula 400 hoods converted to fully functional opening vent versions that was not an un-common dealer option at the time, using parts for the 1968-70 cars vents.
http://www.classicindustries.com/product/firebird/parts/k892982.html
RELATED
http://garage.grumpysperformance.com/index.php?threads/cooler-denser-air.8961/#post-31945
http://blog.streetsideauto.com/accessories/the-legend-of-the-hood-mounted-tach/