It wasn't a tournament. The "unification" is just in reference to the Race-Backlund match.
The NWA champ and W(W)WF champs wrestled each other quite a few times in the past, as Vincent James McMahon (Vince's dad) was a member of the National Wrestling Alliance and recognized their world championship even though his own company Capitol Wrestling Corporation was 'governed' under the auspices of the World (Wide) Wrestling Federation, which he and his then partner Toots Mondt created in 1963 after their initial split from the NWA before he rejoined the organization in the early 70s.
Fwiw, there WAS initially a plan to unify the two titles way back in 1965. The deal was that NWA champ Lou Thesz would lose to WWWF champ Bruno Sammartino and then regain the championship the following year. Vince and Toots offered Lou and NWA President Sam Muchnick $50,000 (with half going to each) to go along with it, but Thesz wanted $100,000 instead plus 10% of the gate or else he would shoot. It caused the deal to fall apart and led to Thesz falling out of favor with Muchnick and the NWA, as he ended up dropping the title to Gene Kiniski the following January. Besides that though, Bruno himself has gone on record as saying he didn't want the title anyway because he didn't wanna take on the grueling schedule that being the NWA champ demanded.