* Engine Management: The biggest hurdle is the difference in engine control systems. The 2003 4.3L likely uses OBD-II, a much more sophisticated system than the 1996's OBD-I. You'll need to address compatibility issues with the wiring harness, computer (PCM), sensors, and potentially the instrument cluster. A standalone engine management system might be necessary, adding significant cost and complexity.
* Transmission: The transmission compatibility depends on the specific transmissions in both trucks. While both might be automatic or manual, the bellhousing bolt patterns and other dimensions might differ, requiring an adapter or a complete transmission swap.
* Mounts: The engine mounts might not be interchangeable. You'll likely need to fabricate custom mounts or use mounts from a similar-year truck with the 2003 engine.
* Wiring Harness: As mentioned, the wiring harnesses are completely different. You'll need extensive electrical knowledge and likely a lot of custom wiring work to integrate the 2003 engine's harness into the 1996 chassis.
* Exhaust System: The exhaust system will likely require modification or replacement.
In summary, while the swap is *possible*, it's not a simple bolt-in affair. It will require significant mechanical and electrical expertise, as well as potentially custom fabrication and a substantial investment in time and parts. It's far more complex than simply swapping in another 4.3L from a closer year model. Unless you're experienced with engine swaps and have access to the necessary tools and resources, this project is likely beyond a beginner's capabilities. Consider the cost and effort before proceeding.
