It's pretty easy if you want to spend the $$$$$ Every engine is just an air pump. Make it suck in more air (big cam, higher rpm,s, supercharger, turbocharger then allow the burnt gases to escape more freely (bigger cam, ported heads, headers, exhaust). Our real problem with 03 & 04's seem to be the light duty auto transmissions. They won't take even the full HP & torque of our stock 5.3 engines so GM "detunes" the full throttle shifts to save the transmissions.
The most practicle way would be super or turbo charging and eventually replacing the transmission with a heaver duty one. That's some pretty big bucks. For less you could sell or trade an 03/04 for an 05 with the 390hp and great 6-speed or heavier duty auto.
Boys and girls and our toys - soon you will see 05's with superchargers so they can get close to or just over 500hp!