Minor vibration @55mph

Hey guys,

Quick question, my dad has a 1973 Dart Sport 340 #matching, 90,000 original miles and when you drive above 55mph there is a slight vibration in the steering wheel that we can not figure out. All drivetrain is original. All mounts are replaced.

Also my 1974 Dart Sport 318 does the exact same thing!

Any ideas of what could be causing this? Has anyone ever run into this before on your cars?