I would bet the car is not a 400 car, not that that matters. My blue car below was an original sprint-6 and I put a 428 in it. I built it for myself. I just don't like to see someone try to sell snake oil to someone unexpectly.
Restoring a 1968 400 4spd, numbers matching Firebird Coupe, Verdoro Green. Larry My old 68. Wish I had her back.