This has been covered before. To make it short it is going to be very $$$. First off the 3.5 will barely fit into the engine bay as it is. Then you have to have a custom exhaust made and have the computer wired up. My best guess is by the time you buy a 3.5 and pay for everything to be installed and fabbed you're probable looking at $10K or more.
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My wife - "We haven't even made a payment on it yet and you're scratching it all up!"