well they do make FMIC kits for the gc chasis. they are jdm but fairly easy to find.
intake is easy, just get a short ram.
exhaust is fairly easy. just get a custom mid pipe made for what every dp and axel back you get.
if your looking to go 3 inch turbo back then get like an hks dp, and have a shop do a custom dp back with like an aftermarket borla muffler or something liek that.
i have a 92 legacy exhaust on my 95. and i know wrx exhaust firts on the legacy, but it does not for the gc.