[Chevelle SS 396 L78]

I know this is an older thread, but my new ‘67 Chevelle SS L78 came with an M20....I didn’t realize the M21 was even available, and I was too naive honestly to know the difference! But, it’s now obvious to me that without spec’ing the M21 on the order the factory put the M20 into the build as the standard 4 speed. Car was an absolute rocket regardless and I never had any issues with the M20. I’d probably opt for the M21 given the chance to go back in time! Still miss the car to this day! Hardly ever see a Deepwater Blue Metallic ‘67 Chevelle these days! Been looking for it for many years!