Loud “Pop” after priming oil pump

Sorry you are dealing with this. I had a fresh motor I built sit for a while- 1 year. I did the same thing... turn the motor over by hand while a friend ran a drill on the oil primer shaft. Sadly I heard the same “pop”.
I removed the heads / intake and all looked good. It sounded like a valve closing so i sent the heads to the machine shop. It seemed one of the valve guides stuck to the valve and pulled its way out. The spring closed it but the guide was moving in the head. The machine shop repaired it with an oversized guide. They checked them all but said even with break in oil on everything- don’t let them sit for more than 6 months before repriming the engine for 20 mins while turning over. ( a bit much I thought)
Anyhow that was a 318 complete rebuild.
Not sure if different with a 383. But gaskets and time is cheap if something is really wrong.

Good luck and I hope all is well with your engine.
Joe