1966 not starting - ideas?

[Dan1970]

Hi all, I am new to the forums and have a 1966 C10 that was originally a manual transmission truck, now has 3-speed floor shift automatic. Recently I was coasting at idle when there was a loud POP noise, and the engine immediately stalled. It was strange, but nothing externally looked out of place, so I cranked it right back up and drove home. About a week later, I cranked it, and while still in Park, the same POP sound and engine stalled.

Fast forward a couple of weeks. Went to start the truck, and the battery (brand new) was dead. Jumped it off from my car, but the engine would only turn about 1/8 turn (but made a single loud click). Banged on the starter, no luck. Replaced the starter, no luck. Tried different positions with (what I think is) the Neutral Safety Switch, no luck. The starter has a good ground, and the S terminal has 12v.

The drained battery leads me to believe the problem is electrical, but could this be a mechanical issue related to the loud POP? Thanks in advance for any advice.