Well, I have not done this swap yet, so take it with a grain of salt here lol!
I have done a 3rd gen LS1 swap, so I'm not totally clueless on the topic though!
You'll need the engine harness back to the ECU, then you'll splice that into the car's harness, hooking up to the power, gauges, fuel pump relay, ignition key, etc.
I would think the Casper's harness would have the ECU connector, or get a good GN harness with the motor and splice it.
I have a complete and running GN I am pulling the drivetrain and all the pieces out of, so that is my plan, just to reuse as much as possible.