After transforming the old west into the cool, cruel mercenary world of the Clint Eastwood Dollar films and defining the style and attitude of the spaghetti western, Sergio Leone made this honest to God American western epic. Once Upon a Time in the West was shot (like his Eastwood films) largely in Spain but he managed to get stateside for stunning footage set in the wonderland of John Ford’s mythic landscape: Monument Valley. Into this dream American west, Leone drops Charles Bronson as his slow-talking, harmonica playing hero, Henry Fonda as a steely, blue eyed killer, Jason Robards as a notorious criminal who signs on with the good guys and Claudia Cardinale as the fallen woman turned beautiful widow who stakes out her claim for the American Dream after her new husband and his entire family have been massacred. Leone transforms western tropes into a horseback epic of bad guys with a heart of gold and an iron engine that reshapes the landscape as its tracks are laid through the wilderness, and he pays tribute to the genre by casting such iconic screen faces as Woody Strode, Jack Elam, Keenan Wynn, and Lionel Stander in supporting roles. Bernardo Bertolucci and Dario Argento co-wrote the screenplay and Ennio Morricone’s operatic score is sublime. It flopped when it was released in 1968 but decades later loving tribute to the myth of the American Frontier stands out as Leone’s masterpiece, a sun-baked blast of frontier opera and one of the most glorious and greatest westerns ever made. But I have one bone to pick with SIFF programmers: as the festival movies into three days at the Cinerama, would it have been too much to schedule this magnificent masterpiece on that glorious screen, which could do justice the austere spectacle and perfectly composed widescreen imagery?Once Upon a Time in the West Harvard Exit, 1:30 p.m. Sat., June 13.