I've got a 67 Fairlane GTA. 390, C-6. I was out fooling around today and romped on it just a little. About 3/4 throttle from a dead stop in "D" . It shifted what seemed like a little early. At the next stop sign it started out in high gear. No shifts. Does the same in "2", starts in high gear. In "1" the car will not move forward or back. Locked trans, does not pull the motor down. Just like it's in park.
Drove it a few miles home. Each stop/start it was the same, starts out in 3rd.
I have rebuilt a sbf C-6 about 15 years ago, but have not been in one since.