I especially wonder if there was any friendship and interaction between Kirby and Steve Ditko, because they had the identical problem at Marvel (with Stan Lee, and not getting full credit and control of the stories and characters they created)in the 1960's. Both Kirby and Ditko were introverted recluses.

In addition to that, Kirby and Ditko were a penciller/inker team on a number of stories from 1959-1964.
Including FANTASTIC FOUR 13, April 1963.

And the "FF meet Spider-Man" story in FANTASTIC FOUR ANNUAL 1, Sept 1963.

And a sequel to that in AMAZING SPIDER-MAN 8, Jan 1964. The last Kirby/Ditko collaboration I'm aware of.

And on many pre-Marvel mosters stories.
Here's "We Met In the Swamp", from TALES TO ASTONISH 7, Jan 1960.

Kirby and Ditko at least must have had an opinion of the other's work, and on their collaborations. But it's entirely possible they did not meet personally, and just worked on each other's pages, given by editor Stan Lee. But I find it hard to believe with the shared experience Kirby and Ditko had at Marvel that they did not seek out each other and have conversations about that. And perhaps other social get-togethers, outside of Marvel.