I think they are somewhat comparable. It was just a hotter motor of the dealership floor. Still very street-able and they made over 12,000 64-66 Mustang with them. Of course they made over 1 million 64-66 mustangs. Mopar was just strong with it's warranty. And used it as a selling point.
IMHO, the D-Dart was a special extremely limited production drag race package. Not really advertised to general public. No brochure mentions.
Now for 1968, the Mopar 340 motor was stronger than even the a 302 installed in GT350's. Those were regular 302 with bolted on aluminum intake and holley carb. I think the 69-70 340 was stronger than the 351W and 351C motors too.
