The Tech Tips section of stovebolt.com has a wide variety of very informative articles. Here is a Tech Tip on the conversion to 12v.

FifthAvenue Auto Parts sells a conversion manual (the owner is a moderator on stovebolt.com).

There is very little reason to change to 12v, unless you need/want 12v devices in your truck. 6v systems need to have good clean connections and grounds. Both battery cables and all ground straps NEED to be a thicker gauge than in a 12v system. If you only need a few low-powered 12v devices, 6v to 12v voltage converters are available.