I checked the vin it’s a factory 5.0 with original engineYou need to dig a little deeper in that harness and look for a part number tag wrapped around the harness.
Have you checked the vin number on the car?
Some titles indicate the number of cylinders
I just don't understand why someone would put a computer into it that clearly was not meant to operate in this configuration.
What did the guy that sold you the car tell you?
My assumption is the original ecu went bad and he put that one in to work just enough to sell the car