I own a CJ2A and an 8N Ford tractor. I agree with the author that charging the battery on a charger is the best option, and using another vehicle to pull start is the next best option. While I have jumped both the Willys and the 8n from a 12v truck successfully, it’s probably not worth risking potential damage to one of the vehicles or injury to yourself.
As a side note I’m tired of messing around with 6 volt electrical systems and plan to switch my Willys over to 12 volt/alternator/electronic ignition.
We also have a CJ2A which I converted to 12V and one nice thing is that the starter does not need to be converted to 12V. The original starter will work fine on 12V. It's the rest of the electrical system that you need to worry about when jump starting it. I personally would not bother with electronic ignition on an old Willys (or converting to 12V for that matter). I am still running points and rarely have to mess with them. I don't believe I have touched the distributor in 5 years and we use our old Jeep regularly. It's a very reliable vehicle, even with 6V and points/condensor.
Disconnect your 12V truck battery from the truck and jump cable the 12V to the 6V tractor posi to ground and neg to battery. If the 12V is fully charged the tractor starter should turn over and start the motor.
One thing that is misrepresented here is the fear of damage to the 6 volt starter with a 12 volt jump start. The windings and wiring in a 6 volt starter are heavier than a 12 volt, so you won't damage the starter unless you crank it for too long trying to start a balky engine. I know numerous cases where 6 volt vehicles were converted to 12, and they did not swap the starter motor! The same goes for the gauges and vehicle wiring; heavier than a 12 volt system. That is because a 6 volt needs more amps (current flow) to produce the same number of watts. Moving on...the danger is connecting 12 volt jumpers to a 6 volt battery that may be damaged. Always wear eye protection and make the last connection to the ground of the donor vehicle, not the dead one.