The 216 in my one ton recently developed a knock.
I thought it was serious, but after further investigation it was indeed a bent pushrod. The truck was recently run on the bad gas that was in the tank before I acquired it, resulting in a few stuck valves and bent pushrods. One rod was bent just enough that it was slapping the side of the block as it spun around.
I replaced 2 bent rods, freed up the valves and added some marvel mystery oil to the crankase and ran some through the carb as well. No problems since.
Not saying that is what your problem is, but it is possible.
Replacing the rods is very easy.
