If the (+) post of the coil primary is on the distributor side, the truck was positive ground at some time in its past. That's the correct polarity for an early 1950's GMC. Ditto on the suggestion to clean and tighten all the battery cable connections and grounds. It only takes a tiny bit of rust or corrosion to severely restrict the current flow, and 6V systems can't stand any voltage drop at all, either in connections or cables. All your cables, hot and ground, need to be a minimum of #1 gauge, and 0 or 00 is better. I like to make my battery cables from arc welding cable. It has smaller wire strands than battery cable, thousands of them, and less resistance. Run a ground cable from the + battery post all the way to the bellhousing or a starter attaching bolt to eliminate unnecessary connections and ground straps between the battery, frame and body to the starter. Smaller-gauge grounds can be installed from the engine block and body to the frame. The starter has the highest current draw, so it makes sense to give it the best ground path possible, Lights, ignition, generator, and other accessories can make do with smaller-gauge grounds. To polarize the generator, touch a lumper wire to the BAT and ARM terminals of the voltage regulator for half a second or so, once you have the battery installed th4e right way and all the generator/regulator wiring connected correctly. Good luck!
Jerry
