* Mounting points: The engine blocks have different mounting points, meaning the engine won't bolt directly into the existing engine mounts. New mounts would need to be fabricated or adapted.
* Wiring harness: The wiring harnesses are completely different and incompatible. A new harness specific to the new engine would be required.
* Computer/ECU: The engine control unit (ECU) is specific to each engine. The ECU from the 4.6L won't work with the 5.4L, and vice versa. You'd need the correct ECU and potentially a re-flash.
* Exhaust system: The exhaust manifolds and the rest of the exhaust system are almost certainly different and incompatible.
* Transmission: Depending on the specific applications of the 4.6L and 5.4L (e.g., truck, car, etc.), the transmission compatibility might vary. While it *might* be compatible, it's not guaranteed. The transmission's bellhousing might not match the engine.
* Accessories: The placement and type of accessories (alternator, power steering pump, etc.) are likely different and require adaptation.
In short, while both are Ford V8s, the differences are substantial enough to make this a very involved and expensive project requiring significant mechanical knowledge and fabrication skills. It's far more complex than a simple bolt-on swap. For most people, it's not a practical or worthwhile endeavor.
