I had the wierdest vibration issue, that was only felt in the steering wheel . . . it would appear once over 55/60mph . . . and, the engine sounded like it was pinging....I checked so many things.. . then FINALLY found the problem . . . When I had restored the car, I used the old pwr steering pump that was with the car . . . HOWEVER, there was part of the bracketry MISSING !!! . . . this is a small block car, so I thought I had everything, BUT - the triangle shaped bracket that goes between the pump & block, was missing. This in turn, allowed the rear bracket to crack, then break. . . in such a fashion, that it was"keyed together", until enough rpm caused it to start to vibrate - AND making it sound like the engine was pinging . . . not sure if that helps you out, as big block uses different pwr steering brackets . . . but maybe something to "steer" you in right direction ?? , , , ,

Good luck