This is what happens after decades of good intentioned mechanics get done monkeying with a truck. The engine when rebuilt was probably just a core to the rebuilder. It came back as a long or short block and was finished out by the shop mechanic or owner using what parts came off the engine that was sent off. The returned engine may or may not be the one sent off for the rebuild. The engine you current have could have been an off the shelf replacement from a rebuilder like Jasper Engines. Your engine very well could have a 6 volt flywheel and a 12 volt starter trying to feed it. It is my understanding that the wrong combination sometimes will actually work but for a very shortened lifespan.
You’ve got this under control and will hopefully soon be back on the road. bartamos is a good helper and as long as he makes his bed checks on time he will get us through this head scratcher.


Martin
'62 Chevy C-10 Stepside Shortbed (Restomod in progress)
'47 Chevy 3100 5 Window (long term project)
‘65 Chevy Biscayne (Emily)
‘39 Dodge Business Coupe (Clarence)
“I fought the law and the law won" now I are a retired one!
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USAF 1965-1969 Weather Observation Tech (I got paid to look at the clouds)