Keep in mind older "classic" geometry typically called for much less seat tube than modern bikes. So, in a *very* general sense, I would not find 56cm in a classic frame terribly shocking compared to 54cm in a modern frame. I can usually find a reasonable fit for myself with bikes ranging from 53-57cm, but as someone said, there are variables that you can not adjust away, so none of that (for me) is true in all cases. If it's cheap enough, try it out & if it still doesn't work, flip it for one that does.