Welllllll, my first car - a '70 Cuda 340 had this same problem! I bought the car in 1976 with 44,000 miles on the clock. When I'd get it on the highway & hit about 65 MPH, the whole front end would feel "light" & start vibrating like crazy. The hood would shake & hit the fenders just like you describe. However, if I stayed in it, the vibration would subside at about 85 MPH. No vibration after 85. I did have the car up to 125 a couple of times - that's all it would do with the 3:55 gears. I always thought this was very wierd. I know it was not tires or U-joints because I replaced them & it did not change this symptom at all.

Sorry to inform you, but I never did find the cause.