here's the first part of this story. it's actually the FF Annual story, I believe, where Sue and Reed get married. this has the X-men, Thor and the Sub-Mariner, as well as a host of villains. just great to see how they change the story to make it fit. all without using the true basis of the story!
what else is interesting is that they used Dr. Doom, the Mole Man and the Super Skrull, all of which ALSO made appearances in the Fantastic Four cartoon that Hanna-Barbera was airing. they couldn't use the Sub-Mariner, but the villains seemed to be fair game.