I don't know what advice I have for you, but I'll share my perspective.

My best friend was completely straight when we dated, and at the time I was more into girls than guys and I was madly in love with her. We spent two weeks dated before she decided that a relationship wasn't right for her, and it broke my heart. We stopped being friends for months. Eventually we made up, and for a while it was a bit rocky at first, but now we're just as close as we were before we dated. She went through a lesbian phase, but even though I still liked her deep down, she told me she only saw me as a friend and reminded me of what happened last time. Then she went back to being straight and was single for a while, and whenever we got drunk we'd make out. Sex came up as a topic and she told me she had been considering sex with me, and we both agreed that we would never do anything romantic, but strictly "as friends". Then she dated until she found her new boyfriend, and now we don't do anything physical or sexual, and we are both okay with that. I'm now pursuing other guys, and we are very close still.

I think in your case, if you are so close you should be able to express your feelings towards her and judge her reaction, and if she feels nothing for you, you could stay close, and if she does have feelings for you, there should be some limits and boundaries set up so that if things don't work out, you won't lose your friendship. Losing a friendship like that is very hard, so if you value her friendship more a romance, you may want to stay as friends.