You may want to consider swapping in a '95- '98 EJ22. They're easier to find, generally more reliable, and don't have the head gasket issues that the EJ25's have. The difference in power is considered negligible by people that have driven both. If you get a '95 EJ22, it's a straight swap. If you get a later one, you'll also need the exhaust y-pipe to go with it, but it bolts into your existing exhaust at the cat. converter. Also, the '95- '96 EJ22's are non-interference so if you bust a timing belt you don't destroy the engine. I've done a couple of these, including one into a '97 GT, and they really are a direct swap. You can find more details over on the Ultimate Subaru Message Board.
http://www.ultimatesubaru.org/forum/