Just use a auto parts store battery cables of the correct length and has the correct ends.
Wiring a truck: There is no plug and go harness. But, yes, buy a harness. You will have to know a little about what you are doing. Do not be intimidated by truck wiring. It is not as critical or complicated as home wiring. The 1941 original wiring has almost no fuses and is primitive by todays standards. You will be updating to fuses for everything, new wire and a new philosophy and style.
Here are the basics to remember: (not everything is covered below, just an overview)
1. Battery power goes from the battery positive post to the starter using a big cable. Battery negative cable goes from negative post to a motor bolt or starter bolt. Braided ground cable attach motor to body and body to frame.
2. The rest of the main power goes from the positive post of the battery in smaller wires. 12Ga usually. The general feed wiring is 14 or 16 ga.
3. You do need a wiring diagram to follow. Every line on the diagram is a hot wire. ***
4. The basic power for components, using a new harness with a prewired fuse box, is supplied by direct fused wires and wires from the fuse box fuses. The fuse box and it's fuses are turned on by the ignition.
5. The "direct fused" wires have a fusible link at the battery end and go to the ignition switch and the headlight switch. Google and study fusible link and size selection.
6. Break it down to: gauges, ignition switch and lights (which may include a dimmer switch, brake switch and turn signals). You may have a heater motor, horn and radio.
7. Everything has a switch.
8. Some components are only on when the ignition switch is on and some are "on" all the time.
9. Power runs to a light, a motor, a gauge and returns thru the body/chassis/frame metal. Not a wire. Sometimes the hot wire is "interrupted" by a switch. The power simply goes in one side of the switch and out the other and on to the component, light, gauge, etc.
*** Just keep this all in mind when reading the diagram. In the diagram, when you see a wire ending in a triangle shape, it means ground. It means the body/chassis/frame metal is physically touching the "housing" or "bracket" of the components by being attached with a screw or bolt or tight fit. As mentioned, this grounding method on an old rusty truck causes many problems. You can see and check the power wires. The "return" thru all the metal and ground straps is very hard to troubleshoot.
If I was going to rewire an old truck, I would convert to 12V. No reason to go nostalgic on that. No one cares. I don't know if the harnesses are sold with 6V in mind. They are 12V harness. Wire may not be big enough. You may be saying you are leaving the old wiring in place till next year. Up to you.
There are not that many wires in your truck. It's not overwhelming and it's easy. Plan the wire routing, dressing, tie wrapping and clamping as you layout and mock it up. Find a nice place to mount the fuse box either upper left of your left foot or under seat.
None of my comments or suggestions are related to the charging system.
