Which K member is better?

The spool mounts are better as they are a "fail safe" design. It was very common with the old biscuits to have the drivers side mount fail allowing the engine to lift up and the air cleaner smack the hood (although there are now aftermarket fail safe biscuits) . I would choose the spool mount k frame over the older one any day.

Faster production at the plant? How does that work, the k frame and engine went in from below as an assembly? I would guess that they were changed to spool mounts as it is a better design, which is why they are used to this day in modern vehicles.