I would try and operate the steering with the front tires off the ground, a good back and forth should have it bleed good enough to work.
If no luck check your pump pressures and make sure the pump is working. I would think most power steering pumps that chevy offers would have enough to move the 400 box.
Mine is a 90s pump so I have no experience with the older pumps.
Someone else did have issues more recent. May look over his experiance and see if it is the same type of pump.
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