I have a 1970 Chevy Nova, 350 CU IN 4 speed. The car starts to vibrate when you hit about 60 MPH. Nothing real bad but enough to bug you. It does not go away or get worse by going faster. I have put on brand new tires, and a new drive shaft. These two items did help but did not eliminate the problem. also if I am going over 60 and push in the clutch it does not cut down on the vibration so I don't think it is in the engine. Any Thoughts??