* Engine Management System (ECM/PCM): The 2007 5.3L will have a significantly different engine control module (ECM) and likely different wiring harnesses. The 2003 LS's system won't be compatible. You'd need to either find a compatible ECM and harness from a similar year 5.3L truck or SUV (which is more likely to be compatible), or employ a standalone engine management system, a complex and expensive undertaking.
* Wiring Harnesses: The engine harnesses are different and won't connect directly. Extensive rewiring would be needed.
* Transmission Compatibility: While both might use a 4L60E transmission, the specific years might have subtle differences that would need to be addressed. The transmission controller (TCM) would also need to be compatible or replaced.
* Accessories and Mounting: While the engine block itself might be similar, the accessory placement (alternator, power steering pump, etc.) may differ slightly, requiring custom brackets or modifications. The engine mounts themselves will likely be different and require fabrication or adaptation.
* Exhaust System: The exhaust manifolds and system will almost certainly require modification or replacement.
In short, while the basic engine architecture is the same, the differences in the supporting systems make this swap a very involved project requiring substantial mechanical expertise, fabrication skills, and potentially a significant financial investment. It's not a simple bolt-in swap.
