1974 Dodge Dart Magnum Swap

My car has the factory balancer that came on the motor and using the factory flex plate. The car doesn't really shake at idle or sitting still. It shakes at high way rpms. I thought it might be the wheels but if I kill the motor while it is still moving it stops shaking. Again factory balancer that came on motor, factory flex plate that came on motor, and a neutral torque converter that was in my darts 904.