Schurkey , steve25 and the other guys that have posted - thank you for the replies. It has really helped me in troubleshooting
Here is short video clip using Schurkey's great tip on shorting the ignition wires. Makes it easy to short the ignition wires
https://youtube.com/shorts/u8cwepyTOcU
This is a video clip of the idle vacuum (engine warm and idling around 750-800rmp). 19" of vacuum looks decent, I think. But, vacuum is bit erratic. Maybe? that can also be a sign of sticky lifters.
https://youtu.be/KPF_T-Tz0Cs
So, I guess the most likely cause of the intermittent clacking/tapping is sticky lifter(s)? Maybe the Seafoam or Marvel Mystery Oil helped free a sticky lifter(s) a bit? And that is why the clacking/tapping is becoming more intermittent/stopping or maybe tiny pieces of sludge are moving inside the lifters?
I tend to agree, it is likely a good idea to tear down a bit and inspect the cam lobes and lifters (rocker arms, valve tips, pushrods too).