I've had the same experience you describe, at the same speed, on my '63 Grand Prix. In my case it was the drive shaft,
but I have had it shortened due to a swap to a TH400.
It could be the wheels too, I once lost a balance weight on the right rear wheel and it had similar vibrations but in lower speeds
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66 Bonneville 4-door Hardtop - Sold - But it has its own thread here
66 Bonneville Wagon - Sold
63 Grand Prix - Sold
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