* Engine Mounts: While the chassis is largely the same, the engine mounts might be different between the 3.5L and 2.7L engines. You'll likely need to fabricate new mounts or adapt existing ones.
* Wiring Harness: The wiring harnesses are different. You'll need to either modify the existing harness extensively (a very difficult task) or source a complete wiring harness from a 1997 3.5L Intrepid and adapt it to the 1999 car's system. This requires significant electrical knowledge.
* Computer (PCM): The engine control module (PCM) is incompatible. You'll need the PCM from a 1997 3.5L Intrepid, and it may require reprogramming or flashing to work correctly with the 1999 car's sensors and other systems.
* Transmission: Ensure the transmission from the 1997 Intrepid is compatible with the 1999 Intrepid's transmission mounting points and bellhousing. A mismatch here will prevent installation. Automatic transmissions are particularly sensitive to this.
* Exhaust System: The exhaust system will likely need modification or replacement to fit the larger 3.5L engine.
* Other Components: Consider other components like the intake manifold, accessories (alternator, power steering pump, etc.), and cooling system. These might require adjustments or replacements to ensure proper fit and function.
In short: While not impossible, this engine swap is a major project requiring significant mechanical and electrical skills. It's far beyond a simple bolt-on swap. You'll likely need extensive mechanical knowledge, specialized tools, and access to wiring diagrams and a suitable parts car (a 1997 3.5L Intrepid donor vehicle would be highly beneficial). Unless you have substantial experience with automotive repair and engine swaps, it's recommended to seek professional help or consider a different solution, like finding a used 2.7L engine or a different, less involved repair.
