In fact, the Cowboys and the Raiders were the final two of the 26 teams in the original post-merger NFL to play one another. Their first meeting was in the season finale in 1974, at the very end of the fifth post-merger season. There were only 8 other NFC-AFC matchups that had not yet occurred entering the 1974 season (7 if you exclude Dolphins-Redskins, who had met in SB VII but not in the regular season). (The others were Jets-Bears, Oilers-Vikings, Bills-Packers, Giants-Chiefs, Bengals-49ers, Rams-Patriots, and Eagles-Chargers.)

Of course, the Cowboys and Raiders were almost annually expected to meet in the Super Bowl, and came very close to doing so several times -- particularly after the 1967, 1970, 1975, 1977, and 1980 seasons, when one made the Super Bowl and the other lost the league/conference championship game. That doesn't even include 1976, when both teams were regarded as best in their respective conference, but Dallas's offense sagged when Roger Staubach broke his pinky against the Bears, and they lost the divisional playoff to the Rams. It also doesn't include 1973, when both lost conference championship games by identical 27-10 scores (Dallas to the favored Vikings, Oakland at the defending and repeat champion Dolphins).

In any event, the "dream matchup" never occurred in a Super Bowl. And it wasn't until 1983 that the Raiders first played a regular season game at Dallas, and the first of three Oakland at Dallas games was not until 1998 (the LA Raiders played there in 1983 and 1986).