I wonder if Egghead ever considered joining the Headmen. After all, he was an evil geniuswith a distinctive head to boot. But perhaps the monicker Egghead would have been too on the nose. Also, whereas the four Headmen were basically unknowns, Egghead had established himself early on as a member of Ant-Man's rogues' gallery. The criminal mastermind went on to lead the Emissaries of Evil (Defenders #42-43).
This pnael from Marvel Feature #5 (Sept. 1972) shows one of numerous times Egghead crossed paths with Ant-Man (Hank Pym).