One thing about the box art: The roofline on the Impala looks kind of square; on a hunch, I checked the Old Car Manual Project. Sure enough, the AMT artist must have cribbed the Impala from this brochure illustration of a four-door hardtop:
http://www.oldcarbrochures.com/static/NA/Chevrolet/1963_Chevrolet/1963_Chevrolet_Brochure-1/1963 Chevrolet-04.html