You have to make it breathe. Heads, blower, turbo(s), nitrous or something. For more ponies you must get more fuel/air in it and get it burnt.
If it were mine (and it's not) I would spend my time and $$$ on fuel delivery, heads, cam & valvetrain, exhaust and ignition to make it as efficient as possible. There's no reason, other than time and money, you can't see 400 hp out of that engine and stiil get the same mpg you are getting now.
Then the question is, will the outdrive survive your driving habits.