Well you should have just made your own intake, and I wouldnt believe the manufacturers claims. You can find a cheap set of headers off ebay for $200 and they will replace the restrictive front manifold and infamous u-bend. exhaust tips are only for looks, and yes you can install them yourself. the clamps may be really rusted on there though. get whatever tips you like, dual/duals are very popular. you may be able to get away with a 3.5 on your basically stock engine, but you would absoultely need a scan/in car/dyno tune with it.