For the ej swap, do I need the ej harness for the alternator, or can I just wiring it using the existing wires from the EA. 

You'll need the wiring from the alternator to the main loom. Then splice this into the existing EA loom.

relatively simple to do. Just make sure you solder and insulate your joints really well

Cheers

Bennie

I did mine using the existing alternator and wiring and have had no problems with it. I did have to shorten the main output stud about a 1/4 inch for clearance.

Just for the information,  the risk in using the old existing wire is that it is not sized for the higher output of the modern alternator.  The stock fusible link should cover most failure modes ok, but not allow the full use of the higher output.