Uhm... 'Res Gestae' by dreamlittleyo (found on LJ or AO3, one of my all time favs, actually the first fic I read on LJ, if you believe it) ... erm, here: http://archiveofourown.org/series/7322

And 'Not Time's Fool' by FayJay (also found on Dreamwidth under the nickname pandarus) http://archiveofourown.org/works/4720



Oh, I just read the caveat 'always been a girl' after the fact... which those stories aren't, though they're both very good and in character writing. Erm, ok, I did an AO3 search of the sam/dean tag with 'search within results' -> "always a girl"

http://archiveofourown.org/works?utf8=%E2%9C%93&work_search[sort_column]=revised_at&work_search[other_tag_names]=&work_search[query]=always+a+girl&work_search[language_id]=&work_search[complete]=0&commit=Sort+and+Filter&tag_id=Dean+Winchester*s*Sam+Winchester

I don't know if that link expires, but you can try it or just go to a fic, click on the Sam/Dean tag, and then enter the search term again, you should be able to find what you're looking for. Hope that could be a little help.

Heaven is High and the Earth is Wide (http://archiveofourown.org/works/523968/chapters/927112)by lexicale is one of my favorite stories: "As wide open as the untamed west is, the Winchesters are always trapped between a rock and a hard place. Sam can't escape the trappings of her gender, and Dean is irrevocably in love with his sister. A western!AU with always-a-girl!Sam." (Sam/Dean)

I read that one ages ago and was never able to find it again. I'm so happy to find it again!