Jerry's theory is very easy to prove if your truck still has a stomp starter. There are two wires on the starter terminal; the battery cable and the main feed wire. Take these off and hook back together with a short bolt and wrap with something to prevent arcing. Now set a 12v battery close enough to the starter so a cables can run to the starer and to a good ground. You now have a 6v truck with a 12v starter and 100% of the voltage is going to the ignition system. This was a trick used back in the days of old rope seals which could really make a brand new engine hard to turn over. After starting and running in with this set up a couple times the engine would then turn over with the regular set up.
