I'm surprised this fight didn't happen in 1979. I've got Norton outpointing Big John fairly comfortably.
Both in their respective primes, I have to pick Ken Norton over Big John Tate. Norton destroyed another Olympian in 1977, Duane Bobick. Ken would jump all over Tate with a vengeance, not fearing him as a devastating puncher. Norton by TKO 4. Remember years later in 1980, Big John was outpointing Mike Hercules Weaver, then Weaver lowered the boom in round 15, Tate stayed asleep after the full 10 count before being awakened by his corner. The date was March 31 1980.
I go with Tate in this one. Norton was pretty much shot at this time. Remember that Norton was slaughtered in the first round by Earnie Shavers in March of 1979. Later in August of 1979 Norton was lucky to get a draw with Scott Ledoux, as Ledoux had Norton out on his feet, hanging over the top rope in final moments of the fight, with only an incompetent referee saving Norton from a KO loss.
With all due respect, I was speaking of both fighters in their respective primes, Norton, 1973-1977, Tate 1979-1980, but good post.
Additionally, 1979 was Tate's best year with wins over Bobick, Knoetze, and Coetzee. In 1979 I've got to go with Tate. Prime vs. prime? I'm picking Norton by decision or late stoppage. After all, he took care of Bobick faster than Tate did (wink nudge).