It can be done, but I would not reccomend it. If you do try it, connect the positive to the battery and the negative to a good engine ground, NOT TO THE NEGATIVE ON THE BATTERY. I've seen batteries explode from this. I've also seen a starter catch on fire and a 6 volt coil explode from too much voltage. Granted the situation was exsessive, the guy was trying to use 48 volts to start a 6 volt positive ground '29 Buick. Your best bet would be to use a 6 volt charger/booster to start your truck.
