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Voltage is like pressure (the positive side of your battery being high pressure and the negative side being low pressure). Higher voltage allows the same amount of current to flow through a smaller wire, just as higher pressure can push more water through a pipe. That's why the wires for 12-volt systems can be smaller in diameter than those in 6-volt systems.
The big difference between plumbing an and electrical system is that plumbing only goes one way, but an electrical system must be a loop, or circuit. A circuit must also have resistance, in the form of a light bulb or accessory. If there is no resistance in the loop, it becomes a short circuit that can destroy the system and the battery and possibly cause a fire (resistance is measured in ohms).
A typical automotive circuit would be one for lighting. Current flows through a wire from the positive pole of the battery (on some earlier cars the polarity is reversed), through a switch, and then to a light socket and bulb. From there it goes through the bulb's filament (which provides resistance) then back through the car's sheet metal and frame, through the battery's ground strap, and into the negative pole of the battery.
The first step is to change your headlights, taillights, dash lights and horn relay to 12-volt. You'll also need to install an ignition ballast resistor between the ignition and the coil, which must be changed to a 12-volt coil. and of course a 12-volt battery has to be installed .
This car has good wiring so it won't be rewired until it's repainted. However, if the wiring is bad--which is more often the case--the best plan is to rewire the whole car from scratch with a pre-wired modular fuse block and wiring harness kit because the pre-'50s cars (and trucks) had no fuses. One bad thing about early electrical systems was the wire itself, which was covered with cloth insulation. Through the years, cloth would deteriorate and the bare wires would touch each other, causing the whole wiring harness to short out and catch fire.
Info was found at this URL and content edited
http://www.lowridermagazine.com/tech/04 ... ire_bomba/

David58bug wrote:… Higher voltage allows the same amount of current to flow through a smaller wire, just as higher pressure can push more water through a pipe. That's why the wires for 12-volt systems can be smaller in diameter than those in 6-volt systems.

Actually this is an incorrect statement. Wire size is dictated by amperage, not voltage…. to a point! More power pushing through a smaller wire will create a voltage drop and heat build up. If we step back and look at what we are doing with electricity, it is actually wattage that is power! Wattage is equal to volts times amps. If it takes a certain amount of wattage to do a job then when you do the same work with 12 volts it will require half of the amperage as it would with 6 volts. This is why you needed larger wiring with 6 volt systems, to carry the amperage. This is also why many manufacturers have been looking at a 42 volt system. We would see about one third the amperage to do the same work. This could reduce the amperage demand of the charging system and further reduce wiring size.

David58bug wrote:…The big difference between plumbing and an electrical system is that plumbing only goes one way, but an electrical system must be a loop…

Hey, I had an idea pop into my pea-brain… This might be a stretch… but, what if we look at water globally? Then in fact it is closed looped! It will eventually return to the source!

Badbugtwo wrote:...Wattage is equal to volts times amps....

And from Ohm's law we know that voltage equals current (amps) times resistance (ohms) - so wattage equals (amps x resistance)(amps), or ohms times current squared
This means that that the power lost as heat in resistance of the wire and connections, or in switch contacts, varies as the square of the current - at double the current, four times as much power is lost along the way. Put another way, when you go from 6V to 12V the effect of any circuit resistance is a fourth of what it was.