If you do fit an S13 rad an S13 owner may well buy your S12 rad off you. In fact if you find someone in the know they may pay both sets of postage to swap
Personally for the small amount of extra space you'll gain I'd be tempted to stick with the standard S12 rad, especially if you're going to mount an FMIC in front of it. You could fit some electric fans and remove the viscous one if space is that tight
