Looks like the Fusion.
Good observation. The line(robots) building the Mustang is the same line that is building the Fusion. The bodysides are "slightly" different on the Mustang so they have their own cells which "marry" into the same line as the Fusion, and even that is more about productivity rather than being radically different. Obviously, there are a few cosmetic differences beyond that, but the same robots that weld/assemble the Fusion also make the Mustang, the weld paths change a bit obviously though. Until the official debut of the Mustang, security watched us closely while we programmed them, even confiscated some guys phones who were suspected of taking pics. After they go through paint its a different story maybe, but the body itself was hard to distinguish from a Fusion.