Under the seats. If it is an SS with buckets they may be tucked in the bottom of the seat sprngs. Rear seat, same place. Any thing that was added above standard or base generated a build sheet, such as an AM/FM Stereo radio. If you are lucky and the assembly line worker was lazy and didn't do their job properly (a warming thought) there may be a piece of trash called a build sheat left in the car somewhere. So a console may have a build sheet under it or tucked under the rug. The build sheet was trash that was suposed to be thrown away. If you were to lazy to throw it away where would you hide it?
By the way the motor and transmission Should both be stamped with the last six digits of the VIN. The motor on the ID ledge part of the block beneath the head, and on the side of the case above the oil pan on the tranny. All date codes sould be within ten days of the build in those years without a labor strilke. Durring a strike year dates are all off as parts were squirreled away up to six months in advance of a strike so that they could continue to build cars with scab labor.