78-81 El Camino and Malibu not only have single headlamps per side, they also have the side marker lamps to each side of the headamps as can be see in this image.
82-87 Have dual headlamps per side, and move the marker lamps to the side of the fender as seen here.
The fenders are different, not only are the side markers moved to the side as seen in the second picture, the way that the header mounts to the fenders is also different between the two.
I ran in to this putting an earlier header on my 83 model. It did ultimately fit after notching the backside of the header to match the mating surface of the later fenders. Not sure about the later headers fitting the earlier fenders.