Tom and a lot of other guys on this Board can answer the flow question a lot better than me but here's my
The early heads were limited to a valve combo of 1.92/1.66. Some used even smaller valves, but I believe the GTO Heads along with heads like the 76 & 092 all had the same valve size.
The production of power difference was likely related to the chamber vol., cam selection, rocker arm ratio, and rpm potential.
The later 2.11/1.77 big valve heads likely benefitted from the larger valve sizes which was made possible by the revised valve inclination angle that began in '67 with the 670 head (most '67 heads remained close chamber with the old valve inclination angle, not til the release of the late '67 061 followed by the '68 heads did the first open chamber heads appear which all had the new valve inclination angle).
Perhaps they had higher power potential, but if they were limited to the same static compression, same cams, same rocker ratios, and same rpm limitations as used with the earlier heads, they likely did not take advantage of the potential the increased valve sizes could offer.
I think what I am trying to say is that air flow was not the limiting factor for stock engines.
The topic has always been too technical for my grasp and this is my basic understanding of it.
It would be interesting if somebody could explain how the bigger valves were useful to a stock engine (if they were) and then how they help with power potential in a non-stock build.