1978? 360 with 318 heads why?

Is this a factory motor? The reason I ask is that from time to time some really strange things come off the line. There was a guy in my home town that had a 73 duster that was a factory 318 two barrel car. No one could understand why the car was as quick as it was until there was further investigation done. It was actually a 340 with a two barrel intake! Seems from time to time when they would run short of motors trhey would make up motors to keep the line moving!