OK, I have looked over older posts, but am going to ask this question any way. I have a 400 that was rebuilt with new crank, rods and flat top pistons before I got it. I do not believe it was balanced when this was done. It had a vibration starting at about 4000 rpm and low compression in one cylinder. I decided to pull it apart and found a flat cam lobe in the same cylinder. I replaced the cam, lifters, heads and timing gears. I verified the harmonic balancer was correct for a 400 and checked to be sure that 0 degrees was at #1's tdc. The flywheel is factory for a 350, but has the 400 counterweight plate between it and the crank. A professional mechanic looked at it when apart and could not see anything out of the ordinary. Now that it is back together the vibration is still there above 4000 rpm in neutral or driving. It is fairly smooth at highway speeds and the vibration is not really noticeable when wide open through the gears. Probably due to the noise and adrenaline. Question #1, am I going to damage anything to continue to drive the car. It is only driven on weekends in the summer time, no serious miles. Question #2, how well do these 400 balancing plates work and could that be the problem?