[GRutter]

As I mentioned, could just be old... problems from the past.
I would focus on fixing what your immediate distributor problem is to get it running.

It's quite easy to pull the distributor again at a later date and see if there is any moisture or signs of rusting again. And if there is, then attempt to locate the issue.

You are doing all the right things.... no water in the oil. No bubbles at the radiator (how did you check that without it running? )(that would be indicative of a coolant / combustion chamber leak)

Don't over-think it or worry with everything it "could be." It's a lot easier to see if you have any other issues after its running.

Did your engine rebuilder also check / surface your head ?