Usually in those fics past Spike is William and all dweeby too.
In the
Bittersweets Series by Herself there is a part quite a way in called
Mrs Grieves and the Abandoned Husband in which William from his human past is transported to modern Buffy and current Spike for a while. But Buffy never liked human William much and Spike loathes him really and he does go back. It's a complicated story, hard to relate here and you really need to read the whole series to appreciate it. But that's an instance of past William coming into the future and present Spike is present also.
"Bittersweets": http://www.echonyc.com/~stax/Buffy/herself/index.htmAn ongoing series of Spike/Buffy tales, meant to be read in order. The first springs from "Wrecked" in season six; the series veers off from canon at that point. These stories are het NC-17, with some slashy flashbacks.There's another story where Buffy unleashes the power of the amulet post season seven and instead of getting her Spike back she winds up with human William. That also doesn't really turn out too well. I don't recall the name of it off the top of my head but it is a good story. I'll try to think of the name of it.