I just re-read your original post and proceed as you wish. My advice is not to complete part of the wiring and then return later to do the rest...just be done with it and rewire everything. It's not going to cost you much more. The front of the firewall is most of the work..go ahead and finish it to eliminate all the gremlins that could show up later. You will have better consistency, too...especially if you are making it yourself. Take your time and take meticulous notes on what you do and include the wiring diagram in your build folder. I set up two centralized ground stations. One is for the firewall forward and the other is from the firewall back. Everything I have goes through these two bus bars that are connected directly to the battery. This gives only two places to go to initially to troubleshoot problems associated with grounds since those issues tend to arise at connection points. IMHO wiring is all about thinking ahead, organization, and planning..the best/shortest route to take, which wires to group/tape together in case something fails, etc. It's thinking...ok I might want to upgrade something (a electric fan for example) later so I want this wire over here where it's accessible and I don't need to take apart a lot of stuff or do surgery..or you might group wires according to functions, the shortest route. That's the challenge with wiring if one wants to do it right.
As was suggested earlier, no way in hell i'm buying pieces and parts of a "year one" harness then cobbling up the other pieces later to end up spending even more than what they are asking for a complete set, but that's MHO. I also know that I would probably procrastinate until something went wrong..then I need to refer back to my notes and start tearing things apart again. My advice is to do it all at once if you're going to do it and know you have new wiring (not 40+ year old wire) that will likely eliminate most or all electrical issues you might experience otherwise....if you do it carefully ad correctly. You CAN do this!