I just finished putting the Painless harness in my 71 FJ40. I was lucky and had the help of an Electrical Engineer/Techno Geek who also teaches an automotive electrical class and has an FJ40. Without that type of experienced, knowledgeable resource, and knowing what I now know about what is involved, I would recommend that you buy one from Coolerman or be prepared to be frustrated, confused, and to spend a great deal of time with the harness spread out on your dining room table or some other such large work surface (hope the Princess understands). Unfortunately, Coolerman has temporarily suspended operations as he moves his residence and shop. That may or may not fit your schedule needs.
You should be aware that the various wiring diagrams for different years all have errors or represent features and options that may be in your truck, but are not accurately reflected in the diagram for your particular vintage. My guess is that Toyota added advanced features to the USA trucks but left them off the wiring diagrams until production volumes for that revised system increased and updates were made for the later vintage FSMs. I found features in my 71 truck and wiring harness that were not in the 71-72 diagrams, but showed up in the later diagrams, sometimes with slightly different color codes and routing (driver vs passenger side alternators, etc.).
Another difficult aspect of the wiring harness task, and one that is not frequently mentioned, is getting it loomed properly so that the proportions are correct and everything fits without strain and with some semblance of order. It requires a great deal of careful measuring, and no small amount of trial and error. Another consideration is that Coolerman already has that part figured out.
I ended up creating my own color coded wiring diagrams so I have a reference of what I did. I strongly recommend that anyone building their own harness from a kit do the same. Even if you rebuild a stock harness wire-by-wire, note the revised color schema on the FSM diagram whenever it has to be changed because duplicating the factory color coding is cost prohibitive if you are only building a couple of harnesses.
Good Luck!