Classic/Antique Car Repair: 1967 GTO vibration, american tire co, vibration condition

Question
QUESTION: Vibration starts above 55 mph and stays about the same up past 70. Seems to come up through body as front wheels and steering wheel seem stable. All stock survivor car.  400 auto with 3:23 limited slip. I have ballanced all 4 tires, replaced u joints and balanced the drive shaft.  What would you look into next?

ANSWER: What brand, size, and type tire do you have on the car? Are the wheels stock?
Brad

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QUESTION: American Tire co. American GT Grand Turbo P225/70/R14 on 14x6 Rally II wheels.  Standard wheel for this car.  Tires have DOT mfg date of 2001 and have 90% tread with no cupping on any tire.  No irregular wear pattern on any tire.  Vibration begins at 50 and worsens with increase speed.  I quit at 80.

Answer
Try something for me. Drive the car at low speeds on a smooth straight surface, say 10 to 20 miles per hour. Do you fee a slight side to side movement in the direction of travel? If you do it is called radial tire waddle and it will cause the same vibration condition at higher speed. Also do the lug nuts on your car have tappered seats where they contact the wheel or are they a cylinder that passes through the wheel?
Brad