First of all, it all depends on how everything is set up. I've not ever done any work on a truck that old (just an old 6 volt single battery 8N Ford tractor), so I'm just going to talk theory, not specifics. If all 4 positive terminals are daisy chained (++++), and all 4 negative terminals are daisy chained(----), you only have a 6 volt system. If you put two 12 volt batteries in, you can cook the electric system because it isn't meant to handle 12 volts. Granted, a truck that old won't have any electronics, so it won't be as bad as running high voltage through a newer truck, but the starter will turn fast and have a shortened life span, the batteries won't charge (6-volt generator and voltage regulator) and you may even have trouble keeping light bulbs from burning out. Now if they are hooked up with 2 positive terminals daisy chained to 2 negative terminals (++--), and 2 negative terminals daisy chained to 2 positive terminals (--++), then you have a 12 volt system. In that case, hooking up a pair of 12 volt batteries (++) and (--) would work...although you may run short on cranking amps. The easiest way to know would either be to look at how the batteries are connected, or to hook up a multimeter to see what the voltage output of the system is. Now as for converting positive ground to negative, just remember that every motor on the truck will have to have the wires switched or they will run backwards. That may not always be possible, because (for example) if the starter grounds through the engine block to the chassis, you would have to rebuild the starter to change the polarity or it would spin the engine the wrong way whenever you turned the key. EVERY electric motor on the truck would give you a similar problem...windshield wipers...washer fluid pump...everything. If they have ground wires, you can make the switch easily...but for motors that ground direct, it gets trickier. Personally, it just seems like it would be a lot of work with no real added benefit....