They are completely different engines.
GM made about five of them.
The first was the old 1955, 265 without an oil filter on it, a rope seal and no block cast in place cam groove.
The second was the standard engine with an oil filter boss, but the early model had a rope for a rear seal. I don't remember when they added the cast in place cam groove.
The third was the standard modern engine with the rubber rear seal, oil filter boss and cast in place cam groove.
The forth was the one that was like the third engine but had a shaft waterpump drive. I think it is reverse cooling, too boot!
And the fifth is the LS1 that is all different except for bore spacing!
And please tell me if I missed one. pdq67