My 69 Dart 2 dr w/ 225 was very smooth. I never drove above 70 mph that I recall. I did get a vibration ~60 mph once, at a higher frequency than a wheel imbalance. It turned out to be rusted U-joints, an easy fix. You changed almost everything in the drivetrain, and multiple cars with different engines and trannys, so I would suspect the front end. If none were rebuilt, it might have been worn bushings. Main suspect would be the strut rod bushings. The suspension was designed for bias tires and tends to want to wander with radials (unless you install offset upper bushings to adjust caster) but that probably wouldn't be sensed as a vibration.
