I believe it was all about the "long" stroke for a work truck. The slant 6's did well, with its long stroke. I had a 67' Dodge HD w/ the slant 6 and grainy gear and it was a work horse.
Agree I had a 1977 Ramcharger with a 360 , it was a dog had 2 360's replaced under warranty , after that put in a 4406 pac block with a torker and 850 holley made a huge difference Truck was a blast with the 440 .