Heavy vibration when turning left

[mosel70]

1970 Chevy C10, stepside, 2wd, 396ci. I had manual steering and installed a power steering kit from Tuckers (pump, gearbox, lines). After installing, it had vibrations when turning. Thought it was a bad new (remanufactured) pump so I returned it and installed replacement. Now good steering straight and when turning right, but it shakes/shimmies bad when turning left, especially at lower speeds like in a parking lot. Good fluid, no air. No play in steering. When turning while on jacks with engine on and off, it seems to turn fine both directions.