I had a similiar problem with my 67 Barracuda for quite awhile. It had a vibration that was only noticeable at 50 mph and above. It was a seat of the pant, as the vibration you described. I always felt it was the driveshaft as my transmission had several tailshafts seals with very little miles. Finally, after many discussions, I took the drive shaft to be balanced, not expensive and as soon as they checked the shaft for runout they found variances of .020 -.060. I opted to have a new shaft built and no more vibration. Just a thought.