I will assume the wire numbers and colors you gave are correct.
1. #9 powers the gauges. The hot terminal of gauges. If there are 12V gauges and 6V gauges you have to "split" #9. You can tie #9 to the 12V gauge, then jumper to the gas gauge with reducer inline in that jumper wire. Any method that accomplishes the task. As you said, I guess #9 can go to the 12V gauge and another wire from ignition switch "ON" to the gas gauge with a reducer in that line. (that's no good if the harness is fusing all gauges). When they started using fuses, and up to at least 1966, gauges were not fused. Power came off the ignition switch "ON" terminal. They would be indirectly fused if the power to ignition switch had a fusible link. Maybe the harness does that and it's not apparent until you follow #9. It may be done via the ammeter instruction in your case.
2. If all gauges are 6V, they each need their own reducer. Some gauges take a different reducer than others. I never had a 6V truck so I don't know.
3. One end of #40 to gauge sender terminal, other end to sender center tap.
4. If you want to learn about wiring and at the same time learn about this harness. Use a multimeter to check continuity of the relevant wires to see where they go. To a fuse panel? thru the fuse to the switched hot bar?...or bypass the fuse panel? Make a note or a drawing of all the relevant wires. Not that many in your case. See if you can figure out what they did and how it all works. The colors and markings tell you what the wires do but not how they do it. Could be enlightening. We are all cooped up anyway so plenty of time.
This does not mean I will stop helping. No problem. But it's pressure for me not to make a mistake.
