Fred Williamson was already in Italy completing work on another film. When he found out about this film he tracked down director David Worth and asked him to give him a role in the film just so his work visa could be extended allowing him to stay in Italy longer because he enjoyed the country so much.
David Worth explained in an interview that the story the poster existed before the movie is "apocryphal." This interview is available in the DVD extras for WotLW release as part of MST3K Volume 26 (XVI). In an on-camera interview Mr. Worth details how he was introduced to the producers and instructed to "make the movie" based wholly on his 40 page treatment.
Director/writer David Worth says that he was hired, shipped to Italy, and told to begin work on the film.... before he ever had a script. He was reportedly shown a poster for the film and told to make a film that would go with it.