Need Advice, Suggestions

Ok, so I just got my 66 Dart GT back, I had a 360 engine and an 8 3/4 rear end installed. When you drive the car, especially at 20-40 mph, it vibrates like crazy!!.. as you increase speed the vibration decreases but it is still there, not quite as intense. The tires and wheels were just balanced, the driveshaft was cut and balanced, The motor was dyno tuned, I am at a loss. any ideas??? thanks, John