If the overspray is also on the firewall, I would say thats fine, but if only the tag has overspray on it, somthing is wrong. The tag was literally the first thing attached to the basic body shell in the Fisher plant. There will be no paint or primer behind the tag. The firewall was painted black prior to body color being applied. Once the car was painted, if there was a huge amount of body color over spray then either Fisher, or GM should have touched up the firewall. If your only talking white on the top faces of the letters, but not on the face of the tag that would probably be left that way. If the tag has a lot of overspray but the firewall dosen't then chances are the tag was removed and the firewall repainted by some previous owner.
As far as comparing tags, the NCRS as an established organisation that people bring their cars to for inspection, has the opportunity to photograph the tags themselves, such that all the images are the same. There is no equivalent organization for Camaros. I've got literally a 1000 67-69 tags, but they have been harvested from Ebay, websites, local car shows, people send them to me, etc. Some I take myself when I get the chance.
As such the images vary greatly in alignments, quality, and angles. I can make some general comparisions and can identify alignment issues but can't go down to the macroscopic levels that the NCRS guys do. That's why I would be loath to call a specific tag a fake, unless it is so bad that it should be obvious to almost anyone if they gave it any thought. I've also not been doing it for years and years. Maybe someday I'll have a much bigger library.