Yeh it is easy to do S13 coilovers.
I made these up by the way:
I've got S13 coilovers now. The only thing I havent looked into is the tie rod ends.
Are the S13 tie rod ends longer or shorter than the S12 ones?
If you use the S13 and S14 lower arms, you won't be able to adjust out the negative camber that this will create. Will make it look good though.