Here are some pics to illustrate the difference. The 4-door hardtop has the large wrap-around rear glass, shorter doors, and no window frames since it is a hardtop.
The El Camino has longer (2-door length) doors, shorter roof overhang, and was designed as a "truck" (well, actually kind of a 2-door wagon with the rear roof cut off...), and thus better proportions and functionality.
The 59 and 60 El Caminos were built off a 2 door 6 passenger wagon chassis, including the rear flank gas fill door and a "banana" gas tank. The banana gas tank was due to the wagon having the spare tire under the floor where a gas tank would be in a car, the tank fit around the tire.
The Pontiacs, including the white 59, have a car gas tank, filled from the rear, so all were likely made up from a passenger car. That maroon 60 looks very well done using the El Camino roof, but it looks like the tail gate does not open? Hard to tell from the pic but is that a gas fill cover above the plate? The El Camino used the rear wagon gate, just covered where the glass would be and had a Blank where the exterior window crank handle was. It unlatched via the same interior latch/door handle that was on the wagon.
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63 Parisienne sport coupe (The Big GTO), black, maroon interior, 409 4 speed; former owner of a 59 El Camino, 63 Corvette SWC, 62 Chev Bel Air SC. 1963- Pontiac top selling car in Canada
Mahone Bay, NS Still not old enough to need an automatic
Also note that the maroon one (actually both, maybe) looks like it was built from a Bonneville with its longer wheelbase and rear overhang. Looks like a long bed El Camino, in a sense, if there was such a thing.
The point is that the El Camino roof and doors have better proportions than a 4-door ht cut down in an attempt to look like an El Camino.
Here's a tailgate on a 59 SD, note no window crank or blank. The guy who just picked these 2 up has already restored a 58, 59, 60 SD and made matching trailers for them.
Thought you would enjoy the pic, even though off topic.
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63 Parisienne sport coupe (The Big GTO), black, maroon interior, 409 4 speed; former owner of a 59 El Camino, 63 Corvette SWC, 62 Chev Bel Air SC. 1963- Pontiac top selling car in Canada
Mahone Bay, NS Still not old enough to need an automatic