[Dick Boneske]

My father-in-law was an engineer at AO Smith in Milwaukee, where most GM frames were built until the 1970's. He said that the movement or twist on a typical automobile from one corner to the opposite is about 4" as the car travels along a road.

Yes, it's normal for your '64 GTO and all other cars & trucks to deflect noticeably when jacked up by the frame. Watch the wings on the airplane next time you take a flight.

Also, be aware that the windshield and rear window glass play a significant role in keeping the body from flexing. Using butyl tape to secure these windows leaves you with much more flexing. Most auto glass shops will not use butyl tape, but insist on urethane adhesive.