Yep, the 'factory' or stock motor would have been the 307.
The only item that would show up on the SPID would be an optional change to that...either up or down. ie: 292-6 cylinder, or 350 or 400, etc.
As for the 4 speed...well there is a chance that the owner took it back to the dealership after he got it to have it (factory/dealer) upgraded to a 4 speed.
The GM dealership would have easy, quick access to all the correct factory parts so it would naturally and expectedly look and perform as an original.
All good
Coley