If you absolutely have to have an aftermarket "cold air" intake, I'd pick up a universal one from that computer auction website for $40.
Or you can open up your stock airbox for more sound.
Either one will be about as effective at making power as anything else you can get on the aftermarket.
The only thing I would modify on the 2.7 would be to swap in a complete dual exhaust off an R/T, if I could find a used one for $100 or so. And that's only if the Charger were in great shape otherwise. You're really better off saving your mod money for your next car.