Vibration on a '63 Vette

There is a high frequency vibration at high speeds that doesn't change when it is in or out of gear or when clutch is in or out. New tires have been balanced and the car has recently been aligned. Any thoughts?

hows your air pressure in the tires? too much and too little pressure will do it.

what, if any, modifications have been done to the car?

I had a similar issue with a 65. I rotated the tires and when inspecting the front rotors, I incidently lodged a small piece of rust between the rotor and hub. Driving caused a slight vibration, this one did vary a bit with speed. Once I removed the rust chunk, all was well again. Probably the simplist and cheapest fix I have had for this car.

tht

It doesn't have any modifications, just stock and old. Although the 327 was repaced with a 350...