I don't know about the 3.9, but you could probably swap in a 3.5. This was actually a common swap in Intrepids and Concordes, because when the 2.7 engine dies, it's so expensive to replace that it can actually be cheaper to swap in the 3.5!
I'm sure the situation would be similar with the Magnum, Charger, and 300. However, if your 2005 is still running fine, honestly it would probably be cheaper to just sell your car and go out and find one with the 3.5.
There are performance mods that can be made to the 2.7 though, to get more power out of it. Pay a visit to www.intrepidnet.com
or just google "Chrysler 2.7 performance mods" or something like that, and I'm sure you'll find some information. Goot luck!