Change Partners and Dance by Dreamweaver also fits the bill. It's shorter at 55,5000 words, but...
Summary: What if when Buffy first arrived in Sunnydale, instead of a souled Angel meaning to help her, she was faced with a soulless Spike, looking to cause trouble and kill his third Slayer?
Runner Up for Best Romance and Best Author at the Sunnydale Memorial Fanfiction Awards! Nominated for Best Long, Best Romance and Best Spike Characterization at both the Spuffy Awards and the Rogue Poet Awards, and for Best Sex, Best Plot, Best Alternate Reality and Best Spike Characterization at the Spark and Burn Awards! Daemons Luminati by Kalima also fits, IMO.
Summary: Can a vampire be dragged, kicking and screaming into the light? We'll see. Spike's a complicated guy, isn't he?
Author Notes: Herself, the most, in bunches. Liz who's unapologetic lust for Spike first made me want to march in Spike-pride parades – we're here, we love evil soulless things, get used to it!
This is dedicated to the yellow balloon. All creatures that can love and grieve have souls.
Story Notes: Season 6 through, oh, I don't know, let's say after she wore the lilac blouse of breaking up and before she wore the gray robe of victimization.
Denial by The Enemy of reality has the slow build happen in another dimension leaving the Scoobies scrambling when Spuffy reappear in their home dimension. Although, again it's very short clocking in at @26,000 words.
Summary: Forced to be close together for an infinite amount of time, it stands to reason that mortal enemies would drive each other insane. But when it comes to a self-righteous, uptight slayer and an irritating, chipped vampire, there is no reason. This story, set just after the end of S4, is full of banter and innuendo, so read on if you are interested.
Also, Riley left to roam the jungle a little earlier so he isn't in the picture.