I know this may sound crazy, but I had an exact same problem about 20 or so years ago. Tried everything I could thinhk of,. and a few other tricks from different folks.
In the end, what the problem turned out to be was a mix up in the cam I got sent. It was a Marine cam for a boat engine which turns in the exact opposite direction to an automotive engine.
Like I said, could be a long shot, but if nothing else works, take another cam, stuff it in, and try it.
It sounds like a timing problem, my old man would probably say , lift the distributer, turn it 180 degrees, drop it back in and crank it over. It'll either start, or back fire.