The 67-69 were the ones that needed the increased Caster, however it was corrected in the second gen . As far as smooth and comfortable, they were all smooth and comfortable, well maybe a little stiff in the bumps... but they were built to handle in corners.
The Caster makes front wheels return to center, the classroom example is forks on a bicycle, they are bent forward giving it Caster. Offset shafts were more commonly made for the 67-69, but were made for rally racing. So to answer your question, Just good marketing. Remember you don't see very many people Drifting a 47-59 Chevy Truck!
