That 10% number may be about right.
When I looked to buy a 69 Indy Pace car, I looked for about a year and looked at at least 20-25 advertisements in Hemmings, Autotrader, and other online sources, (ebay was not really established for selling cars at the time I bought my car). Of all the cars I looked into, only about 3 or 4 of them still had the numbers matching motor. Many had CE replacement motors or "correct" motors, while the majority had just any old chevy SB in them. The ones that did have the numbers matching drivetrains were rust boxes or really beat up. Mine was the only pace car that still had the original drivetrain from the factory and the original sheetmetal (qtrs, floors, trunk, etc.) with almost no rust. Also, this was back in 1999 before the muscle car craze and the staggering amount of fraud that exist today.
Even today I still checkout all the 69 Camaro pace car ads, ebay, and Barret Jackson, and it is quite difficult to find these cars with the original drivetrains in them. I guess these cars were not bought with the notion of keeping them stock and instead were used and abused and if the motor or tranny went...pop in another, no big deal. I bet the original buyers of the first gen Camaros had no idea of what these cars would become 35 or more years later.
So, if you have a numbers car, appreciate it and take care of it.
So, 10% seems about right as far as