I would say,it is a SW, because it is in the book (Bruckner).
OK, that’s no explanation. Usually we say, it is a SW if there’s some Italian participation in the project (Studio, director, producer…)
In thius case it is difficult, cause the director is British (Terence Young), it is filmed in Almeria, some sources say, it was co-produced by Italians (don’t know who). For Bruckner this is enough to call it a SW. I can’t say, how big the Italian participation was.
I would call it a Eurowestern.