Whatever the 5th digit in you VIN is, would mean that engine came in the car from the factory. Are you getting the VIN from the left fender or paperwork?
The 'optional' refers to a better item being installed at the factory beyond the low end base car. I6 was base ($0), the 289 was an upgrade option (+$$$$).
Just looking at the engine may not be enough. The older I6 cars came with 4 lug wheels, smaller brakes, smaller rear ends, and a few other things. Over the many years, almost anything could have been changed.
The 'optional' refers to a better item being installed at the factory beyond the low end base car. I6 was base ($0), the 289 was an upgrade option (+$$$$).
Just looking at the engine may not be enough. The older I6 cars came with 4 lug wheels, smaller brakes, smaller rear ends, and a few other things. Over the many years, almost anything could have been changed.