68 Barracuda with an externally balanced LA360. Been trying to narrow down the cause of the vibration at around 2200 RPM. Replaced the all metal motor mounts with correct rubber mounts, which helped a bit. The harmonic balancer was old and worn so thought maybe that could have been the issue. New one helped slightly but vibration is still there. Vibration happens both in neutral and in drive. What else should I be checking?