This is great advice here and what I would also recommend. Re-purposing factory connections and the PDC is the cleanest way to go. I’ve done stand alone efi from Holley and it’s a great setup. That being said, a lot of you stock under hood wiring will remain. The ecu itself will be deleted but there are some basic things it provided that you’ll need to either get from the new ecu or will need to piece together like your 5v reference signals for map, tps, vss etc.
Also, I use alldata for all the wiring diagrams and it has everything you will every need to trace down wiring and thin out what you don’t need.