George ang Gilly,
maybe I'm completely wrong and this is not a first hand experience - but nevertheless:
The hydraulic lifters grow when under a no load situation, like laying in the box. The lifters shrink when under pressure, slowly and not much, but they shrink.
That is the way they are compensating more or less valve clearance automatically (of cause they are supported by oil pressure in the running engine)
If you get new hydraulic lifters into the rockers (new or old) and tighten the rocker arms - the valves will open because of the grown length of the lifters. Just let the head sit overnight and check the valves again.
A buddy from a german forum reported that he changed his hydraulic lifters and afterwards the car didn't start! No compression at all.
So he took off the head to check for any damage, but everything looked well. He mounted the head and the next day: engine started and ran.
If this is nonsense - please forgive me