The spool mount isn't lighter, or better. In fact, the early style K member is a better piece. The spool mount was designed for faster production at the plant.
Careful throwing the BS flag. The 73+ K frames were known for coming apart in a real good handling car. I've replaced a few myself with the 67--72 style. Never the other way around. I'd still use one over the /6 K-frame though. I am running a 73+ in the 67 Barracuda, but it has been reinforced by Firm Feel.