I have a 440 in my 1971 GTX that the engine was casted 1-28-70 s/n 0G246189 and F 440 on the pad. The car and engine was rebuilt in 2010 and driven 600 miles over 10 years. I purchased the car unseen and had a prebuyers done on it. I have been working on the car over the last year and one thing it has is a vibration over 3500 rpm clutch pack forward. What are the differences between these two years of engines and could it be due to something as simple as the wrong year harmonic balancer.