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?