I think you'll love both of these fics that I'm about to refer. They're both written by Eurydice, who is above and beyond one of the best writers for the Spuffy fandom EVAR. The first fic takes places in 1940, is rated NC-17, and is AU all human. It's called By Rook or By Crook. The second is also NC-17, takes place at some point in the '40s, and Buffy and Spike are still Slayer and Vamp. It's called Rhapsody In Oil.
By Rook or By CrookRhapsody In Oil(just so you know, both of these fics are complete. And now I'm going to go read them for the millionth time because I just made myself nostalgic.)