Possibly The Many Shades of True Heroes,
http://www.tthfanfic.org/Story-10158/LMouse+The+Many+Shades+of+True+Heroes.htm Summary: Clark is accidentally transported to Buffy's universe, where Lois is a Slayer and somebody's after The Key ... Pairings? Spuffy. Buffyverse Kal-El/Buffyverse Lois.Can't find it in the archives here but I think it was posted on the boards.
