As noted Pony's are originally positive ground. They only have a simple field regulated generator. The field wire should come from the light switch. The light switch has two positions for two different positions for charging. One is direct to 6v power for high charge, the other has a resistor for low charge. If your field wire is connected directly to the ignition switch it will cause overcharging.
The battery needs to be hooked up while running to operate the distributor ignition.