The Met is going to have a rough time coming up with a Plan B after calling off the John Galliano exhibit, we’re told. There’s nine months until the famous “first Monday in May,” when the Met Gala usually takes place. But that’s an extremely tight timeline. A source tells us that a significant amount of work had likely already gone into “John Galliano: Horizons,” which was slated to open in May 2027 with the star-studded Met Gala, which is a vital fundraiser for the museum’s Costume Institute. That work would have included everything from coming up with a concept and research to sourcing archival pieces and tracking down important items…