Sure, you can change the exhaust as much as you can stand from the standpoint of noise and federal compliance. If you want a side exit ahead of the rear wheel you'll be before the muffler somewhat, so will likely need to drop altoghether or use a flatter oval-section muffler like a Flowmaster. If you go that route, be sure to incorporate some type of heat shield to keep from melting your carpet.

As for the intake, something like a good cone filter or the Morgan-Tec intake will allow less restriction granting a mild power increase. But any increase in power from more air intake means proportionately greater fuel consumption, especially in the case of forced induction from a supercharger or turbo.

The surest way to get more power and economy is to replace your engine with the direct-injected version of the 3.5 from an '04 Axiom or Rodeo.