I've always been curious as to why Hemi Darts were hard tops and not post sedans. There wasn't really a 'Cuda post-type car either so Plymouth would have had to field a Valiant. And despite hard tops having somewhat less overall rigidity, they didn't come with frame connectors either. It really was a curious choice.
My belief is that it was a marketing decision based on the "Win on Sunday, sell on Monday" adage. Let's be honest, '68 Dart post sedans and Valiants are not the sexiest cars out there so I imagine they made the concession to use the hard tops and fast back 'Cudas for that reason. In 1968, the 'youth market' was just getting established as a viable consumer base. I'd wager that in 1965, a 'racy' appearance probably didn't matter as much to people like it might have three years later. A lot changed in a short time.
Sure the post cars are cool now but back then my guess is the general buying public viewed them as cheap, throwaway transportation and wouldn't be caught dead in one. Either way, the '68 Hemi Dart might be one of the coolest vehicles ever produced by the Big Three so I'm glad it came to be regardless of what platform was chosen.