Get Dan's book are do a lot of research.
Only "quick swap" engines would be a 3.2 and 3.5. To save yourself a lot more work, when shopping for a motor, you need one within the years of 2002-2004 to fit your 2003. Between 1998-2001, and 2002-2004 they deleted some things such as the EGR valve, oil coolers and other small things, and changed wiring harnesses etc. You can make a off-year model engine work but it requires more work like swapping timing gear(s) and intakes etc.
You'll need a complete drop too. Power steering, alt, a/c, engine mounts (literally a complete drop), the engine harness, some hoses, the transmission dip-stick tube; to name some big things to help you in the mean time search around.
Engines in the LX platform cars (Charger, Magnum, 300 etc) are the same identical engines. Intakes were redesigned, that's about it. But as said, your barking up a more difficult tree which will require a lot of wiring hassles, maybe a few mechanical timing issues to cooperate with different electrical components.
That said, if the 2.7 in your car has been well run through it's life time, never "granny" driven, it should produce excellent power as it won't be improperly broken in or carbon'd up. My 2.7 floats, literally smokes all my buddies fancy new Lancer's, even my friends la-de-da Mustang (highway we're talking here), 240KM comes quicker then any of them can touch 180KM. There are small things to do to pep it up such as true dual exhaust, cold air intake, bored out throttle body, port and polish heads, LX platform intake, bored throttle body, even a 300M SPECIAL PCM, which will even bring a 3.5L to life.
You'll gain from a 3.5 swap now though. Drop the 3.5 in and loose the waterpump worries, gain power and have a 2.7 in running shape to turn around and sell or keep for spare. Best part, you'll have 2.7L final drive gears on your tranny, so that 3.5 just got a bigger kick in the pants!
Lots you can do, just a matter of what you can get done or wanna spend.
Last edited by UPGI2AYDD; 05-20-2012 at 03:27 AM.