With correct size cables and solid ground, a 6v system is just as reliable as 12v. The main advantage is any 12v accessories you might want will be an easy install after the conversion. I kinda like being able to plug the laptop into the cigarette lighter, and I opted for 12v wipers; but there are devices that "step up" from 6 to 12v if you're worried about originality.
I'm running a 6v starter in a 12v conversion and the truck starts fast, since the starter is spinning the engine over faster than it idles. If you go this route, never crank more than a few seconds at a time or you'll be buying a new starter anyway.
You'll also get small benefits such as brighter headlights and the availability of 1157 bulbs. Since 12v systems run half the amps of a 6v, it's less likely a short will cause a meltdown or fire.
