I think it will just bolt on. I bought the stuff to do this on my '96 but never did it. It sounds like you are upgrading the wheels too, which you probably would need to do of course to clear the larger brakes. The rear should also be upgraded, otherwise it is going to have much more front bias than it already has.
Why are the Outback brakes larger? I think it is because the wheels and tires are larger diameter, so larger diameter rotors gives it more mechanical advantage in stopping. I run Outback wheels on my '96 legacy in the winter, and the brakes still work fine, but you kind of realy have to step on them moreso than with the smaller tires.