How do I jump a 6 volt truck with a 12 volt truck

There is only one correct way.

Before you connect any jumper cables, check the 6v polarity since many 6v systems were positive ground and you have to match the batteries, not the vehicles. Also, you must be sure there is no metal-to-metal contact of the two vehicles.

Next, you need a second person. If you hook up the jumper cables from a 12v to a 6v vehicle and then go get in to start it, you will blow most of the 6v circuits. That's why there are no working fuel gauges on the 6v trucks.

Turn off every thing on the 6v vehicle.

Have the second person connect both cables to the 6v battery being sure to observe polarity.

Have the second person connect the 6v positive cable to the positive post of the 12v vehicle. Do not connect the negative cable to the 12v yet.

The driver of the 6v vehicle turns on the ignition. As the driver hits the starter switch, the second person TOUCHES (does not clamp) the remaining negative jumper cable clamp to the negative post of the 12v battery. This is only done while the 6v starter is drawing a full load. If the driver (6v) releases the starter switch, the second person immediately removes the negative cable clamp from the 12v battery.

There is no problem with the 6v starter operating on 12v. Many vehicles are using 6v starters on 12v systems.  The problem is that you might "fry" the 6v generator, voltage regulator, 6v battery, and any other instruments or accessories on the 6v vehicle with 12v.

The 6v starter motor "absorbs" the additional voltage so it does not harm any other 6v circuits, BUT ONLY WHILE IT IS CRANKING THE MOTOR.

I repeat this important instruction. Make contact with the 12v battery ONLY WHILE THE 6v STARTER IS DRAWING FULL LOAD. Break contact with the 12v battery instantly if the driver releases the starter switch, either because he quits cranking or because the 6v motor starts.

There is always a risk of a battery explosion. I'm not an engineer but I expect the 6v battery is more likely to explode. I've had 12v batteries explode and it was always the one getting the jump. The second person should wear safety glasses and protective clothing. Safety First!

I grew up with 6v, positive ground vehicles. I learned to do this correctly and never hurt a 6v system. I will consider converting each of my 6v vehicles to 12v in case I have to jump one of them when I am by myself. This may be the most practical reason of all for converting to 12v.

Of course, being able to add certain 12v accessories - radios, tape and CD players, spot lights, driving lights, etc. - can be another reason for having a 12v electrical system.