The DB has Alfredo Santacruz as the hired hitman, Buck Garrett. He seems to me to be Spanish stuntman, coordinator, actor identified in another forum to be José Luis Chinchilla. I can not find the Spanish credits to check if he is in the crew. Neither the Italian or German have a maestro d’armi.
Patricia is identified in another topic by @JonathanCorbett as Maria Villa. Also spotted: Adolfo Thous - poker player who relinquishes his seat to Garfield along side José Riesgo, Emilio Rodriguez - villager who instigates the hanging, Guillermo Mendez - Mangosta henchman, Joaquin Parra - Garfield henchman, Rafael Vaquero - Garrett hitman, Ricardo Lillo - bank manager and the actor known as ‘Nazzareno Natale’. Has he ever appeared in an Italian only production?
I did not find Spartaco Conversi and do not know Marisa Porcel.