"Endless Summer" is basically a documentary, not a narrative story that has narration or voice-over, the commentary was not competing with or had to be woven into and out of the dialog.
"The Shawshank Redemption", "Amadeus" and "Forrest Gump" are solid examples of how to weave VO/narration into and out of a film. In all three the narration was done first (in "Forrest Gump" the entire narration was filmed on the park bench set by Tom Hanks as Forrest) so that the spaces needed for the VO could be integrated into the shots or a B-roll shot list compiled. The approximate time for each piece of narration was noted before shooting began, and sometimes even read or played back, while the shot that would have the VO was being filmed. This way the editors, both visual and audio, do not have to cram or expand the narration to fit the space or use "bad" edits to fill the time needed for the narration to take place. You should watch/listen to the DVD commentaries of all three films as the directors discuss how the narration and dialog were worked out prior to production.