Usual , the title 'bestfriend' among a male and female is just an excuse for.... I like him/her, i talk with him/her a lot and trust him/her a hella lot, i'm just not ready to be with him/her, or don't know if he/she likes me back. (bestfriend title may stay even if feelings were long gone)
Go for it, there's no telling. If she doesn't like you back and acts weird after you 'expose' your feelings, then forget her, not a good bestfriend to allow that slide pass.