Learning by bad experience is not the cup I like to drink from. I feel your pain. Read this:
https://www.stovebolt.com/ubbthreads/ubbthreads.php/topics/1387191/2.html

Other than lifters, hard oil sludge in the valve guides or wrong valve guide material, maybe.

I agree with Mike. I replaced mine on a 235 (solid lifters) for bending. Worked well.
If the lifter has collapsed and is sticking hard, it may allow the push rod to fall out of the rocker arm cup (engine running).

"Adding CFM to a truck will only help at engine speeds you don't want to use."
"I found there was nothing to gain beyond 400 CFM."