It's more of how open cockpits provide natural advantages than how truly "lenient" the 6-view in F4U(or any other plane) was. I will post a pic proving this. Look caregully in the two cases you've provided F4UDOA when the cockpit is in the "would-be-closed" position(in yellow lines).
When the cockpit is open, the lofty space provides enough movement room for the pilot with loose straps and reared seat to deflect/lean his torso left and right, which would provide him the "6-view" you say. Effectively, his head movement will pass through the maximum space provided if the cockpit is closed.
Now, consider how it would be when the cockpit is closed. The pilot moves about 5 inches left or right and his temples will be pushing against the glass. If he attempts to twist his torso, lean oneside and look at 6 O'C, his nose will press against the glass.
You can also see how the pilot in the second pic is really straining forward, providing the optical illusion "he has a lot of room". He isn't sitting comfortably back. He must be either reaching forward and operating something, or checking out his gunsight. That sort of "leaned-forward" position comes out typically only when the pilot is in battle, flying level and aiming at something. If he sits back, his head would be restricted to the cockpit glass just as in the first picture. Even when he is leaning in such a manner during flight, it still would restrict his head and torso by about 10~12 inches room.
In my conclusion, this sort of setting with the seats reared up and pilot head position high is done with the premise that the cockpit will not be closed during the sortie. In other words, from the beginning the pilot probably requested his seat be that way to the ground crew, because he was gonna fly with the cockpit open.
Why was he gonna fly with the cockpit open?
Because, when he didn't his 6-view was gonna be restricted heavily.
Cased closed.