The stories that are the closest to what you want are the EURYDICE stories
LEGIONS OF TRUE HEARTS and the sequel
A SYMPHONY OF ECHOES, that were mentioned above.
UNBRIDLED BRUNETTE is starting another story that looks like it will be another masterpiece. Make sure you read
THE GIRL WITH FLAXEN HAIR. The story is posted on this site and on EF.
Here are a few others that are have a strong William in the past thread:
THE PAST IS PAST ----- by: PANDORA ANNEhttp://www.fanfiction.net/s/2758441/1/The_past_is_pastSet during 'Touched' In between when Buffy is kicked out and Spike comes to find her, Buffy is sent back in time to 1879.
The Seven Circles of Samsara-----by Evenstarhttp://spikeluver.com/SpuffyRealm/viewstory.php?sid=6821A Christmas Eve wish by a fountain in Rome leads Buffy to a strange place where she's reunited with her soul's mate. However circumstances show her that all is not as it appears to be. This is a Buffy/William, Buffy/Spike fic, but ultimately Buffy/Spike.
IMITATION OF A MAN----- by: DARKAPPLEhttp://www.livejournal.com/tools/memories.bml?user=darkapple&keyword=Imitation%20of%2 0a%20Man&filter=allThe events of "Dead Things" plunge Buffy and Spike into Spike's human past in an
experience that transforms their relationship. I LOVE, LOVE, LOVE THIS STORY!
A Cricket from California --- by Elsa Frohman http://www.sand-in-my-shoes.com/vd/elsacfc.htmlA Christmas story. Buffy gets an unusual Christmas present and travels back to 1875 to meet a young William.
This one is another one you might enjoy. Although this is more in the time travel vein.
Things Present – Things Past------by: By Estepheia, Marceehttp://www.allaboutspike.com/fic.html?id=187Synopsis London 1880, A Watcher fears for the Life of his Slayer. Sunnydale 2001 Buffy is alive but longing for simpler times. A wayward spell pulls Buffy into the year 1880, where she soon learns that - no matter what - things can always get worse. Meanwhile, Maeve, the Slayer from 1880 meets the Scoobies. But Buffy and Maeve are not the only time travellers; Spike and Xander.