Someone dropped a 1969 GTO engine in your 1968 Firebird at some point over the past 45 years. That's the most logical explanation.
Pontiac sold 107,112 Firebirds in 1968. There was no shortage of these cars, and no issues with them "moving them off their lots".
If it was a warranty claim, it would be an SR coded block.
The ONLY story I can think of where dealers changed engines to "get them off their lot" was with the 1969 ZL1 Camaro, where a dealer was stuck with a few of these cars due to their $8,000 price tag (double the price of a 1969 Camaro). They yanked the ZL1 427 out and dropped in a 350 or 396 to get rid of them. The dealer then sold the ZL1 engine separately.
On a different tangent, some Plymouth dealers, left with unwanted 1970 Superbirds by 1971, and high-perf cars being harder and harder to sell, yanked off the nosecones and wings and sold them as regular Road Runners.
