I need some help with an old Dodge truck.

IF, the condenser is good, 6v condenser will work fine (to start, at least) on 12v.
I have three six volt mopars, and a 6v Ford. When I want to MAKE them start, I get them spinning on six volts, jump them with a twelve volt battery, and pull the 12v off immediately when it starts. Some quickstart helps too.
Six volt systems are all (almost!) Positive ground. Jump with the twelve volt battery OUT of the car, pos to pos, neg to neg.