Did you do this while the engine was warm?
I don't know too much about the early six hydraulic lifters, but seems they may be too tight.
When I adjusted them I had the engine idling and would back the adjuster screw off until they would "click", then tighten the adjuster screw until the click just stopped, then 3/4 of a turn more.
Never had any problems doing that. But someone who knows better should be along shortly.
