Don't forget Nan Smith - she has a great series of stories on alt-Clark. It starts with "How I Spent My Christmas Vacation." From there, branch out (if you haven't already) to her other stuff. It's all fantastic. There's a great series guide for her stuff on the Archive, if you haven't already seen it -
Series Guide for Nan Smith Also - classified as an elseworld story but very alt-Clark-ish is Pam Jernigan's
Tryst (and its sequel, "Merry Trystmas").
'Toc