These days, it can sometimes seem like the line between gallery shows and museum shows is blurring. As museums try to shed their image as lofty temples of culture and capitalize on the excitements of contemporary art, galleries seek gravitas and substance. Occasionally, galleries mount exhibitions that are so substantial they can fairly be described as museum-quality. Two such exhibitions can now be found at the David Zwirner Gallery (537 West 20th Street) in Chelsea, and together they remind us that the history of post-war art is being revised as we speak.