I would hate the idea of a friendship to a girl for such a long time...
I don't believe men and women weren't made to be just good friends.
There's always the sexual thing which disturbes such a relationship.
I once heard that when once you change from "good friend" to "partner" status you loose some of the privileges you had as a friend. She won't be that open to you anymore. She will avoid many topics which could damage the relationship.
What would be positive is that, since you know each other that good, you can avoid those many tiny mistakes a fresh couple does often...
As I can tell from my sister, she has some "friends", and she could never imagine that it would go any further with them. She likes to talk to them because they understand her that well.
Sometimes I think that girls, just don't want a guy as a partner who understands her well. Because it would be too smooth... No surprises, no big romantic evenings, no more embarrassing silence when you meet.
But that's only a theory
If I was you, I would tell her how you feel. At last you're close friends and to close friends you should be honest and open... there should be not exception of the rule!