Here's the KING KONG VS GODZILLA plot description from Toho. While it condenses the story considerably, it leaves no doubt about who Toho says won the final battle.
This was published in Toho Films a few months after the movie was released in 1962. IMO, the fact that it was written close to the movie's creation and at a time when Honda, Tsuburaya, Tanaka, Sekizawa, etc were still alive adds far more credibility to it than any "retroactive" thinking on Toho's part (I should also add I've never seen Toho officially contradict what was written in 1962).
Hopefully this will put this matter to rest.