My buddy bought a 70 Challenger a couple of years ago and just completed the restoration. The car shakes and vibrates pretty badly at about 55 mph. He had new u-joints put on and had the driveshaft balanced. Now he can drive a little faster but it still shakes badly at around 65mph.

The automatic tranny was rebuilt, 440 six pack motor rebuilt and balanced. New wheels and tires balanced, front end aligned. It has an 8 3/4 rear.

Any ideas as to what could be causing this?

Thanks, Rob