Slant six?
Good luck with the swap. I love my EG rig but it truely will be easier to make things fit.
WRX has the same differential size as the SVX, 3.73 R160. The center differ is a 1.1:1 so the front is a 3.90 in a WRX. I Would just go with a nice R200 unit or something of the sorts if your going to be shuffling everything to the back. Also be aware of the carrier mount it may be a huge PITA to get to be true.