Rocker arm adjustment

[Dan Myshrall]

I've got a 72 SB350 with 101500 miles on her. The lifters were replaced 5000 mi ago. (25 years ago)... she sat in a barn for a long time. There's a clacking when not accelerating that drives me mental. Sounds like rockers to me, like maybe they were poorly adjusted from the get go. Local Chevy guys said get a spare rocker arm cover and cut out the top, and adjust them while running. Is there anybody out there that can give me some input as to how I should go about this... or if I'm going in the right direction. She runs strong... but I don't want to damage anything. Thanks in advance.