Good luck...
As an example, my 323 wiring is routed from the nav desk through the sink cabinet base in the head to the starboard lazerette allthe way to the transom then under the cockpit deck and up the center binnacle tube and into the instrument pod.
I pulled a backbone cable through the same route fairly easily, used a seatalk1 to Seatalk ng bridge to connect old instruments to new. I’m still running a hybrid system with Quantum radar and a newer MFD with ST60 wind, tridata, and autopilot instruments in the pod. The old communicates with the new, and the ST60 data shows up on the MFD screen as well.
I really didn’t have to special order any cable to specific length and didn’t cut any of the cable set that came pre-packaged as a bundle with the assorted “T” connections. I heard and read a lot of stories about not being able to connect the old to the new, including having to replace transducers and that the old could in no way talk to the new, but so far everything I have connected is working.