He was cast as the lead, recorded all the songs (although they were never used as Gordon MacRae took over when Sinatra walked off the set) in Carousel which I assume was being filmed in 1955.
Yes, I've seen the wardrobe test shot in the (I think) Carousel DVD bonus material. I'd say even in that one Sinatra would not have come across as the handsome leading man type that MacRae could not help but exude in his portrayal of Billy Bigelow. His wardrobe test shots suggest to me that his Billy would have been more of a raw boned sewer rat carny rather than the robust, well-fed specimen MacRae provided. I believe that approach also would have been closer to the Charles Boyer portrayal in the original French movie, Liliom, on which Carousel was based (and, imo, the "right" way to play Billy). That said, I totally understand why, in retrospect, we might have preferred Sinatra to play Sky in 1955, particularly since his version of Sky's big song, Luck Be a Lady, became a true signature number for him yet he doesn't sing a note of it in the movie. But, again, that song became a signature number for him much later in his career, after he had transitioned so successfully from the scrappy underdog, hapless kid brother persona into more of a 'take charge' tough guy persona in his movies and in his live appearances.
One thing I like about Brando in this movie is that, because of its "stage bound" production concept, this comes closer than any of his other movie roles to showing us what Marlon Brando The Broadway Actor might have looked and sounded like. And I'm even including his role in Streetcar Named Desire, where he took to the more intimate nature of film acting like a champ. But in Guys and Dolls, despite it being a genre he would not be known for on Broadway, we see a Brando "performing" as to a live audience in many ways, dancing, singing, broader gestures, etc. I just think that Brando is a lot of fun to watch, too.