97 legacy gt wagon swapped with 04 wrx with turbo

[jeffneedshelp]

Hello so I am quite new to swapping engines and I would like a little insight on the wiring portion. I know my engine and transmission would fit into the car, but I have been hearing some mixed concerns with the wiring portion; I was told that I could just build a new wiring harness with a 8ft dog tail or I can use the old wiring harness form the 97 gt, But the problem is that I don't have the 97 wiring harness and I have no clue where to start. I have the engine in my garage and the chassis on standby but where would I start with the wiring? do i get a 97 harness and plug into the wrx or should I run 2 ecms