I did this mod a few weeks ago and it is somewhat difficult but not impossible.
3.5 HO Plenum from a 2002-2004 3.5HO equipped car. (Intrepid R/T, SXT, Police, 300M, Concorde Limited, Prowler (yes, Prowler!))
3.5HO PCM and a SKIM module, matching SKIM key and lock cylinder from the donor car. If your car doesn't have sentry key, you must add it if the donor car had it.
Upper engine wiring harness. You will need a harness to connect the manifold tuning valve and short runner valve to the PCM. Grab this off of the donor car as well.
If you got your PCM from a 300M Special or Prowler, you will also need a set of transfer sprockets from the car to correct your speedometer. They have a slightly lower gear ratio (3.89 vs. 3.66) and are used to improve acceleration. These are a PITA to install..... trust me, I did it!
The performance gain is quite noticeable and worth the trouble. You might be able to get a new PCM from the dealer but be prepared to pay!
Notes on SKIM or Sentry Key Immobilization Modules: The SKIM is a part of the SKIS (sentry key immobilization system) and must be wired into the car if you do not have it. There are only four wires to wire in. You need a ground, always on battery (B+), ignition on battery and PCI BUS. To tell if you have the system, look at your key. SKIS keys are gray and fat, Non-SKIS keys are thinner and black.
This mod shouldn't be attempted if you aren't familiar with wiring as it involves quite a bit of it.
After this is done, complete the package with a suspension upgrade and Autostick!
HP figures depend on the PCM you get.
R/T and Police: 244HP
SXT and 300M: 250HP
300M Special and Prowler: 253 HP + more aggressive shift scheduling
Police: 130MPH (LOL!)
SXT: 108 or 118MPH It's one of the two, I forgot!.... any input?
300M: 118??? I'm not sure..... someone up for a little midnight "fun"?
300M Special: 152MPH!!! I haven't verified this..... I only have H rated tires!
Prowler: 130MPH? I'm not sure on this, the figures vary from 115 to 130.