First of all: Barquero is not a spaghetti western. It's a full-blood American western, shot in Colorado, directed by a Hollywood veteran, Gordon Douglas, who had made several action packed westerns in the previous years, such as Rio Conchos (1964) and Chuka (1967). The presentation of its protagonist as a taciturn cynic surely bespeaks some influence of the Italian western, and so does the location, near the Mexican-American border. As Philip French put it, the Italian western had pushed the Hollywood western further south, and made the western hero more cynical, and less verbose.
Bandit leader Jake Remy (Oates) and his gang of cutthroats want to cross the Rio Grande as soon as possible after they have pillaged a town and massacred the entire population. Van Cleef lives in a small town near the river and is referred to by the townspeople as "barquero" (Spanish for boat man); he has built a barge to connect both river banks, and is captured by three members of the gang, sent ahead by Remy. But he is saved by a friend, Mountain Phil (Tucker). Since Remy needs the barge to bring a wagon load of silver to the other side, a psychological battle begins, with Van Cleef and the townspeople on one side, and Oates and his gang on the other.
Most probably Barquero was supposed to do for Van Cleef what had done for Eastwood: launch him as a star back home after a successful trip to Italy. For the occasion, some Italian aspects have been smuggled in. Oates' pot-smoking villain is reminiscent of Gian Maria Volonté's El Indio from For a Few Dollars More and with the priest being portrayed as the most irritating (and selfish) of the townspeople, even some of the typical anti-clerical feelings of the Italian western seem to shine through. But it's obvious that The Wild Bunch was a major influence as well. Not only Oates was one of the bunch, but with a bloodbath early on in the movie, a more subdued mid-section, and a particularly violent conclusion, Barquero's structure is also very similar to Peckinpah's movie.
Overall Barquero is a nice blend of Italian and American influences. Both large-scale action scenes are ferociously violent, and show some nice directional touches (Oates and Mathews having a relaxed conversation inside while outside the massacre goes on), but they lack some fluency, as well as the sense of immediacy Peckinpah created. The mid-section feels rather protracted, with most of the action (or the lack of it) confined to one spot on the Rio Grande, but the film redeems itself in the last twenty minutes, with an Oates losing both his patience and his senses, and a grand finale, played out more or less (I'm not joking) as a naval battle.
Barquero wasn't very successful at the box-office and critical reception was lukewarm. Unlike Eastwood, Van Cleef only became a real star (and eventually a legend) in the course of the years, with the revaluation of the spaghetti western genre. He's as unyielding as ever, but his taciturn character suffers a little from this static mid-section and Oates and Tucker seem to have more interesting roles as, respectively, the hallucinating maniac and the lively, ant-eating mountain man. There are two female roles too, for Marie Gomez and Mariette Hartley, and especially Hartley's character is interesting (if not particularly uplifting). She's one of townspeople and even though he has the lusty Gomez to sleep with, Van Cleef dreams of having a night with her. Hartley has noticed his attentions, and offers him her charms in exchange for his help (and boat) when her husband is captured by Remy. Van Cleef sees through her plans and rejects her, but then she becomes aware of the fact that she is attracted to him too. In the true style of the brute, the boatman takes her into the woods, and then rips off her clothes. Subtle it ain't, but it works, like the movie.
* Philip French, Westerns, Aspects of a movie genre, Exeter, 2005