Here's the lowdown on what is involved in swapping your Intrepid from a 2.7 to the 3.2 or 3.5 HO engine. This also applies to the Chrysler Concorde.
You must match your year range with your engine. If you have a 1998-2001 Intrepid/Concorde, you will need a 1998-2001 3.2 or 3.5 engine. For 2002-2004 models, only the 3.5 Magnum or 3.5 HO can be used.
Engines must be complete as possible. They need to have the starter, alternator, a/c pump, power steering pump, a good engine wiring harness, upper plenum, and motor mount brackets. Just ask for a complete "drop out" when calling a boneyard.
The engines can come out of any LH model car, which includes Chrysler 300M, LHS, or Concorde. Intrepid ES, Concorde LXI, and Intrepid R/T are also good donors.
The 2.7 ECU will operate a 3.2/3.5 engine without problems. The 3.2/3.5 wiring harnesses will plug right into the 2.7 car and computer.
The transmission will also bolt right up. All LH models were equipped with the 42LE. You will gain a gear ratio advantage as well, because the 2.7 vehicles had a 3.89 ratio, and the 3.2/3.5 models had a 3.66 ratio.
Here's what you need to connect a 3.2/3.5 into a 2.7 car. Everything is pretty much the same parts wise for 1998-2004, with the exception of a few parts. The 2002-2004 models have an IAT sensor in the air intake hose, whereas the 98-01's do not.
Air intake to throttle body pipe
Oil Cooler Lines
Power Steering tube (pump to high pressure line)
Power steering reservoir
Power steering suction tube (reservoir to pump
Power steering reservoir bracket
Transmission dipstick tube
Transmission cooler lines
Upper/Lower Radiator hoses
Torque converter bolts
Radiator (2.7 does not have oil cooler, 3.2/3.5 does)
You can get all this stuff used, or new from the Dodge dealer, they are relatively cheap, except the radiator. I would purchase the radiator at a radiator specialty shop to get the best price. You can retain the original 2.7 radiator, and install an aftermarket oil cooler (hayden brand is best), but I suggest you have your original radiator cleaned and thoroughly checked for leaks or other problems.
It sounds like alot, but it is still cheaper than installing a used 2.7. The 3.2/3.5 engines do not have the notorious problems that the 2.7s do, and will last and last, if taken care of.
A/C: The 2.7 a/c lines can be re-bent and re-used, or you can get the factory 3.2/3.5 lines from the boneyard with your engine. They will bolt right in, and make for a much neater look. You could always buy them new also, if you wanted to spend an extra $200.
Exhaust: The 2.7 exhaust has to be slightly modified to work. The passenger side pipe has to be cut right behind the post-cat O2 sensor and shortened 2 1/2 inches, and re-welded back together. The hangers have to be re-positioned slightly to get them perfect.
93-97 3.5 engines: 1st generation engines are not compatible for the swap.
This is pretty much a bolt-in, plug-in swap, if you have all the correct parts.
This swap works only with a complete, used 3.2 or 3.5 engine. I suggest replacing the spark plugs, oil pan gasket, timing belt, and water pump before installing it. Its not absolutely necessary, but will save you maintenance work down the road, especially if the engine you get has over 50K miles.
If you install a 3.2, you will gain about 30HP, a 3.5HO, about 45 HP, over the 2.7. Not to mention the lack of the 2.7 problems.
Post your swap questions/pictures/results/concerns here, I will be glad to help!