The only thing from the 240 engine harness that connects to the car is the gauge cluster, and HVAC stuff. Otherwise, get a pin out for the EJ harness you're using and go from there.
I looked into a swap like this when I picked up a cheap EJ. If you want to use the stock mount locations on the car with the stock location on the engine, it will sit far to forward in the engine bay. You'll also need to mod the trans tunnel as the Subaru trans is "thicker". Also keep in mind that you'll need to fab up a rear shifter stay for the Subaru trans as it has an external linkage type setup.
If I think of more I'll let you know, but at the time I had the hardware, but I didn't and still don't have the skill.

 

Not being sarcastic, but I wish you lots of luck. Not saying it can’t be done, but not sure it will be worth it especially since you mentioned you’ve at a loss when it comes to wiring. This definitely won’t be a plug and play type engine swap, and will require a lot of time and money. Like mentioned you will need to fab engine and trans mounts and do some fab work to clear the trans tunnel, also you will need to get a driveshaft custom made.

My suggestion would be to sell the Subaru engine and trans and find a Nissan KA or SR engine and trans combo, though they’re getting tougher to find, it will save you a bunch of headaches because they will bolt right up and there’s a ton of support on other forums as well. There’s companies like Wiring Specialties that make wiring harness for s13 and s14 engine swaps, that make things much easier.