I believe Kirby never intended for the Eternals to exist in Marvel continuity.  He gave in to editorial pressure and included SHIELD agents, as I recall, but the "Hulk" who appeared in the only "team-up" issue Kirby did was a cosmic-powered robot built by college students working from a comic book.  Had Kirby had his way, I don't think the Eternals and their story would ever have had anything to do with Spidey, the Avengers, or the rest.

It was Mark Gruenwald (a fan-turned-pro if ever there was one) in Thor Annual #8 who incorporated the Eternals, Celestials, and co. into the Marvel Universe proper, and generally "upped" the level of cosmic power in the books by a few notches.  Of course, Dr. Strange kept introducing cosmos-quaking heavy-hitters on a routine basis, as well...