Which K member is better?

I don't know. From what I remember those solid mounts would fail and send the fan into the radiator among other things. And of course the motor would shift. Funny, I have owned seven 73-74 dusters through the years and never had a K-frame fail. Although if you push a vechile out of it's intended use. Like trying to turn it into a road racer or full out drag racer with 700 hp. things break right?