I've come to regret Plymouth's extinction. Their engineering of the Valiant was impressive, of course. They also had the blessing of the parent company coupled with resources.
What made me admire the Duster more was learning that they did that almost in secret. Apparently they wanted to go head to head with the established sports cars of the time. It seems they also feared that Highland Park would say, "Stop it!" if they got wind of it. So they developed it on a shoe-string (about ten million bucks, if I remember right), kept it as quiet as possible and introduced it into their line-up before anyone upstairs realized just who they were marketing to.
They get the respect of enthusiasts for squeezing a more curved window into standard Valiant doors. It's not something that we would even think of. Yet difficult to do with few resources.
And I don't know if this is a true consensus or if it's the feelings of a few enthusiasts, but I get the impression that a Duster with a 340 V-8 would stand up well to a Mach 1 at about half the cost.
I've never owned a 340. But I've owned a few 318s and one 360. From my experience with those, I'm as confident as anything that the 340 was gutsy, fast and durable.
Those Chrysler V-8s were fine engines. That high pitched starter noise was kinda weird. But it seemed to work okay.
