Check for voltage drop through the key/switch circuit.
The reason that just about all 12V cars originally used a ballast resistor was so that they could bypass it for starting, therefore have a hot spark during cranking. 6V cars don't have this luxury
Also as above, make sure all cables and connections are good and 'big' and make sure battery is up to snuff.
On a side note, my Dad drove GM and other 6V pickups for quite a few years when I was young, and they started, grinding over slow, on some VERY cold days.
You have to keep them tuned
You have to have good quality parts
clean cap and rotor
AND A GOOD CONDENSOR (cap) in the distributor. "New" does not mean 'good'
If you are using resistor wires, consider going to solid wires, LOL
What shape is the engine in, compression, burn oil, etc?
All the battery power goes through the amp gauge. Poor connections there will rob the system of amperage. You can't afford to loose amperage on a 6 volt system. Might also want to do a load test on the battery, just to be sure its up to the task. 
