Friend in HS
390 4 speed 67 Cougar
Another guy 428 67 Mustang Fastback
Another guy in my Mustang club 67 Shelby GT-500KR 427 actually with 3x2 carbs - car looked brand new 10 years after production
I had a 30 minute ride in a Shelby AC in about 69. Las Cruces, NM on I-25. 427 with a single huge Holly. Towards 600hp in a 1300lb car IIRC. In third gear, the owner would goose the throttle up/down and whomp 60 to 90 to 60. Chirped the tires at 60 doing this. What whiplash! It loped along at 110 in 4th gear. We never got it into 5th. Between the wild engine and open cockpit it was the noisiest car I've ever been in.
The 67 GT-500 with a 427 you mentioned likely would have pushed 650hp. This was NASCAR level power of the day.