I'd take pictures of how every thing is routed and clamped then rip it all out. No need to label existing wires. New harness wires should be marked or color coded. Remove gauges and speaker grill for better access. Get some brass thumbnuts from the hardware store to remount gauges. Lay the harness in the truck with wires going to the general area where they attach. Lay it in open clamps where applicable. Start any where you want and take your time. Inspect for correct hookup as you go. Meaning after you wire headlights, compare it to the diagram. Same for gauges, etc. After everything is hooked up go back and tighten clamps and dress it up where needed. Don't install battery until everything is installed and compared to the diagram.
Take your time, do it right and have fun doing it. If you get frustrated back off and get your thoughts together.
Good luck.
