General Motors' 1968 "Muscle Car" Styling

Among the things noteworthy regarding American cars in the 1960s were the "Pony Car" and "Muscle Car" categories.  The former were Ford Mustangs (hence the "pony" name) and similar competing cars.  Muscle Cars were high-performance automobiles.  This Wikipedia entry cites a huge number of models as being of the muscle variety, so to my mind isn't very useful.  For what little it's probably worth, so far as I'm concerned a Muscle Car is a 1960s American automobile smaller than a "standard size" car, larger than a sub-compact, with a powerful motor and having a hardtop coupe body.

Examples would include some cars using General Motors' redesigned for 1968 A body hardtop coupe format.  In the Gallery below I present some of these, indicating what steps GM stylists and marketers used to make them visually distinctive.  As usual, images are of cars for sale unless otherwise noted.
Gallery

The entry-level basis for those muscle cars was the Chevrolet Chevelle 300 coupe.  Not a muscle car at all, and with a B-pillar.

Rear quarter view.  An attractive design, though the shape of the quarter windows somewhat restricted outside views for back seat passengers.

Chevrolet's muscle car version was the Chevelle SS 396 as seen in this Mecum auctions photo.  Muscle cars often got aftermarket wheels, so tend to ignore these as not necessarily being part of the factory designs.  The SS is distinguished largely via a black paint stripe along the lower body that then draped over the front of the hood.

Quarter view of a different SS showing the stripe.

Moving up GM's brand hierarchy, here is a Pontiac GTO shown in a Barrett-Jackson auctions photo.  Besides the distinctive two-part Pontiac grille, the quarter windows are larger and differently shaped than those on Chevelles.  No paint stripes here -- a clean body aside from chromed wheel opening surrounds.

But some GTOs received the faddish (in the 1960s and 1970s) vinyl top coverings (Barrett-Jackson image).  GM A-body coupes lacked sculpting in the C-pillar zone that would have provided a strong basis for defining the vinyl coverage.  The result can somewhat arbitrary, but Pontiac stylists used the bottom edge of the backlight and a subtle kink in the quarter window framing to anchor the vinyl.

Next we have Oldsmobile's 442 (sometimes 4-4-2) in another Barrett-Jackson photo.  Its muscle car status is revealed by that vertical stripe.

Mecum photo of an Oldsmobile 442 Hurst Coupe.  The stripe here begins along the quarter window and then follows the fender line.  The trunk also is painted black.  The quarter window shape in similar to the Chevelle's, but slightly more rounded.

Buick A-body hardtop coupes were sometimes given muscle car- type mechanicals, but there was no distinctive model name.

Shown in these Mecum photos is a Skylark hardtop coupe.  Quarter windows have a pointed shape.  The vinyl placement is less related to body sculpting than on the GTO shown above.
So GM's A-body muscle cars for 1968 were made distinctive via decal and paint-pot styling.  I notice two air scoops on the hood of the GTO, but they are small and so far as I can tell, any other unique metalwork is subtle and limited.