This was excactly my problem too, tapping hydraulic lifters, sometimes quiet, most the time not, even followed this too:
http://chev235guy.blogspot.com/search/label/Hydraulic%20Lifter%20Noise

Ended up pulling the cam, running it into Delta cam and having it ground for solid lifters. I am so much happier now. It sounds just perfect, slight mechanical sound, just music, not one valve tapping its rear off.


1953 Chevrolet 3100
261 cu inch, sm420, 3.55 rear, torque tube still,omaha orange, still 6 volt, RPO green glass, side carrier spare, all done
In the DITY Gallery
Video of the 261 running

1964 GMC 1000
305 Big Block V6, sm420, the next cab off restoration