How rare is a 68 GSS?

Here's what I REALLY REALLY do not understand. The 66-7 B bodies were well underway with the 440, and of course 383. The 67 A was a complete re-design from 66, earlier, and Ma DID put a factory 383 into the 67 A although "marginal." Poor manifolds, etc. WHY did they not spend a bit more time on the base design and figure "in" the HP 440 and decent manifolds right out of the chute?