Uhh, not that this was same thing . . . but - on my 70 Sport Satellite, small block, I installed the power steering pump with everything that was with the car - BUT, it was missing a plate. That allowed the pump to vibrate - caused vibration in the steering wheel, but not the rest of car. Up to about 60mph, all was good, over that the steering wheel would really vibrate in your hand. Turns out that without the plate, the pump was vibrating, and had cracked the mount in such a way, it was keyed together, until you got up the speed/rpm . . .then it would come loose and vibrate !! . . . I went thru the usual, balance front wheels, exchange wheels front to back, check out front end . . . wasn't til I was looking at another small block with power steering that I noticed the missing plate !!! . . .

It's the triangular plate in upper left of pic (Thanks Hoffman's winners circle). . .