The passing game has changed dramatically since 1978 when the NFL Rules Committee made two major changes: (1) DBs could not hit receivers beyond five yards (prior to that, receivers were chucked all the way downfield before the ball was thrown); (2) OL could extend their arms (prior to that linemen could not engage DL with arms extended, giving the advantage to the defense). At some point - I don't recall when - the head slap was also outlawed (again, a big advantage to the DL). The passing game has also gotten more complex over the years, with the chief architects/pioneers being Don Coryell and Bill Walsh (who was an assistant coach with the Bengals). Prior to '78 it was rare to see guys pass for over 3000 yards or complete 60% of their passes. In the '60s-'70s the AFL/AFC was more wide open than the NFL/NFC but eventually with the rule changes everyone threw the ball more and more effectively.
Today's game and the game before '78 is really apples and oranges.
Any hockey fans? Consider the NHL in '83 when Gretzky and Edmonton were an unheard of scoring machine. Back then the league-wide goalie save % was 0.875 and league-wide GAA was 3.76. Today those numbers are 0.915 and 2.55. That means that for the typical team averaging 30 shots on goal per game, it would score, on average, 99 less goals per year today than in '83. In Gretzky's era, teams combined for 7.5 goals per game; today that figure is 5.1 gpg. Games which ended 5-3 back then are ending 3-2 today. If you figure about 75% of those 99 goals would be go to the forwards on the top three lines, that comes to about 8 more goals per man per year. Today, Gretzky would probably top out around 150-160 points, not 200+. Goalie equipment, goalie technique (the butterfly save, which keeps goalies lower in the net), more aggressive neutral zone play and more widespread blue line speed have led to less scoring.
That is why it is hard to compare Ovechkin, Crosby, Kane and Stamkos to the top scorers' stats from 20-25 years ago. Same holds for QB yards and passer ratings.