One of the proposed titles for this film was "Frankenstein Vs. the Giant Devil Fish". In fact, one of the lobby photos features the title monster doing battle with a giant octopus. However, that scene was deleted from the film. Ironically, the sequel, Furankenstein no kaiju: Sanda tai Gaira does feature a battle between Gaira, the Green Gargantua, and a giant octopus.
The film's story originated from an unused 1962 screenplay entitled King Kong versus Frankenstein, written by King Kong (1933) special effects technician Willis H. O'Brien. In the story, Dr. Frankenstein's grandson was to have created a 20 ft tall monster from the remains of animals and the monster would end up fighting Kong. The story never got past the screenplay, thought concept art depicting Kong and the Frankenstein monster exist. The screenplay was given to John Beck who in turn sold it to Toho who made Kingu Kongu tai Gojira (1962) and Frankenstein Conquers the World (1965). Willis O'Brien was never paid for his contribution.
This was a Japanese/U.S. co-production. The versions released in Japan and the United States were nearly identical with the U.S. version being trimmed by about 3 minutes. The international version, prepared to export outside the United States and Japan, was actually longer and replaced the "earthquake" ending with one in which Frankenstein battles a giant devil-fish (octopus).