By the way I heard a clip once from one of Sinatra's later concerts, where he told the story of how he 'tried' to give Brando some singing pointers while they worked on this film. According to Frank, Brando basicaly blew him off.

While his singing in 'Guys and Dolls' is basically pleasant enough, it's clear Brando was not suited to musicals and I believe this was both his first and last.

Barbra Streisand briefly considered Brando (as well as Gregory Peck) to play Nick Arnstein her first movie, 'Funny Girl' years later, but that didn't work out. I read recently that Sinatra was also considered by the producers, but for one thing Streisand didn't like him and also I truly believe that he was one of the few stars who would've been capable of upstaging her.
In the end, Omar Shariff got the part.

Good thing, because I'm not sure how the chemistry would've worked between Babs and either Marlon/Frankie.

Apple