I bought a 65 Nova SS with a title a couple years ago off a classic car guy on ebay. The title VIN# matched the door vin, and it matched the trim tag under the hood but one of the numbers looked to have been grinded from an 8 to a 3 (on the trim tag).
I ended up taking off the drivers front fender and found a hidden VIN which did not match the title or VIN on the door. Needless to say, I found out someone took a 6cyl car and put SS VIN and title. Have to be careful these days. Since is the same time period, the 63 may also have this hidden VIN but not 100%.