I have a 99 without cruise, and some pretty hacked up wiring from previous owners.

I have an 03 donor with cruise.

Will the chassis side harnesses swap so i can add cruise and nice wiring all at the same time? Or is it not that simple?

 

Unfortunately, not that simple, specifically around the O2 sensor connectors for a simple swap, your cam position sensor vs VVT parts, and the immobilizer key stuff.

if you go looking for an NB1 harness, there are at least 2 versions, one for Fed and one for CA-spec. It’ll directly impact the O2 sensor pigtails, at minimum. Unfortunately, I can’t remember the part number which was which. I think I discovered the difference on redlineautoparts.com and then confirmed it in some old PDFs.