Old freewheel removal
It seems like the first question to answer here is: freewheel vs. cassette. The, assumed, age of the wheel and being a 6-speed does suggest that it's a freewheel. However, even if it is an original wheel from the late '70s it is not out of the question that it could be a cassette (at least according to Wikipedia).

Looking at the parts, I see several clues that make me think cassette:


The three rivets visible in the second and later photos.
The nine splines around the outside of the freewheel/cassette body.


Sheldon Brown has a page called Freewheel or Cassette that may help. The biggest differentiator that he mentions is that:


On freewheels the extractor splines do not move when it is spun backwards, but
On cassettes the lockring splines do turn when it is spun backwards.


He also mentions that many cassette hubs have a distinctive bulge, like the one on the image below, on the cassette side of the hub. Sheldon says that if it is there you almost certainly have a freehub, but that the absence of a bulge is not a definitive sign of a freewheel.



Once you know what you've got you'll know if the the strategy is to unthread the freewheel body or to find a way to loosen and lift off the cassette. It is possible that the cassette has worn into the freehub body, like this image from Sheldon:



and will need some fiddling to remove. This article and video may help.