I saw pieces of GBU yesterday while doing work at home (Cinemax, 380 minute version, not letterboxed), and absolutely the film was designed from ground up to coordinate with music, not just the end scenes. First example is Angel Eyes arrival at Stevens' home, how he turns his head exactly in time with the music; very likely that the music was already written and the scene timed to the music. Another example might be Tuco smoking Blondie's cigar found in the campfire. I'm sure there are more....
Yes, the film is very much like an extended music video. The film score books contains a quote from Wallach who says the only scene they played music on set for was the beating scene (since the actors needed to mime singing the song)
