The Cowboys' five straight 11-win seasons happened 1976-80, when the Colts were in the AFC. The Vikings' last double-digit win season of the '70s was in 1976, which was also the only season during 1976-80 when the Redskins made the playoffs.
Between 1976-80 the Steelers, Raiders, and Dolphins made the playoffs 11 out of a possible 15 times and won four out of five Super Bowls. The Oilers were also more successful during that stretch (three playoff berths, two conference title games) than either the Redskins or Vikings. The AFC most definitely ruled the roost during that time period.