^I think I recall Godzilla showing up in an X-Men
issue set in Japan. He fought Lockheed, IIRC. I thought at the time it was just a pop-culture joke, but I guess it must've been a crossover with Marvel's Godzilla comic.
It's interesting how Marvel treated most of its licensed tie-in properties as part of the overall Marvel Universe rather than separate realities. They folded in ROM, Micronauts, GI Joe, Transformers, Godzilla
, even Jack Kirby's take on 2001
. There was even a Marvel Star Trek
comic in 1980 where a simulation of Marvel's Dracula appeared and Spock referenced characters from that comic as real historical figures.