You think that Luffy would have shared this dream with the crew off panel? I think it would be unlikely that he hadn't considering that his crew is basically at the same level of friendship that his 'brothers' were. But at the same time robby has a good point when saying puffy only tells people stuff when they ask and considering he blurts out he wants to be King of the Pirates all the time then why would they ask? So its possible but I think it would be weird for Luffy not to tell his friends his most wanted dream of whatever the crap he said.
I do believe that he said something other than "I'm gonna be King of the Pirates" because it doesn't make sense for his brothers to react that way considering he had already said it and that Shanks implies he said something other than that to him. As to what he said who knows ¯_(ツ)_/¯