Surely puts emphasis on the level of certainty, whereas really usually puts emphasis on scale. For clarity, you could say "That looks really bad" and achieve the same meaning as "That really looks bad." You could use surely interchangeably in the latter instance, but not the former; "That looks surely bad" doesn't make sense.
Incidentally, the word surely isn't often used in English in this context, and really is much more common. I mostly hear the word surely in posing a negative statement or question, like this:
Surely you don't intend to stay up all night finishing that paper?
That sentence is saying, "You don't intend to stay up all night finishing that paper, do you?" Here's another example:
Surely he'll be back soon.
Even though the word "surely" means a high or absolute certainty, in this sense it's used to express uncertainty.