Exhaust cfm flow

I see all the aftermarket heads for sale now but it makes me wonder one thing. In the 70's racers spent so much time on high port plates for the exhaust. Everything I have read talked about the exhaust flowing so badly. So why is it that from what I see the new heads are not really that much better than the old 4 barrel heads. Sure they are lighter but I saw a set of factory 4 barrel heads on Ebay I still wish I would had won the bid on. The flow numbers after the high port plate and intake work was as high or higher than any brade new head designs. Am I missing something????