The LS swap is one of the most widely documented platforms for replacing a factory powerplant with one typically from another manufacturer than the vehicle being swapped. This is largely in part to the abundance of LS/Vortec engines available at your local junkyard, and their ability to maintain operation through some really serious stuff, even in factory form.
Having built harnesses since 2018, I have heard almost every impossible combination, and that is because the LS swap is such a simple feat that many choose to tackle it from their own garages or backyards and there is SO MUCH information on the internet that it is really easy to get turned around or confused on what engine you have sitting on your stand or in your engine bay.
Two great websites I constantly use for reference are OnAllCylinders.com and ICTBillet.com, both which are geared at making the LS swap even easier to understand. Below I have generated a listing of all of the current RPO codes for the numerous engines available to help in identifying what you actually have. While some machining can transform a 5.3L Vortec to a 5.7L displacement, that does not make it a LS1, because the LS1 RPO carries much more than a change of displacement. This is critical in determining what variation of engine harness you need, because some LS engines will have sensors located at different lengths from the rear center of the engine which may lead to some routing challenges.
Below I have created a table that will help you determine the correct engine RPO you have. If you are still unable to determine based on the table, you can look on the block for casting numbers and search OnAllCylinders.com for more in-depth details of each of the RPO codes.