Clean your grounds.
What size cable are you running between the starter and the battery? Ground leads too.
6 volt needs larger gauge wire than most of us today are used to seeing (at 6 volt, it's got less it can afford to loose, so losses hurt more).
Even with all that working right, don't expect a 6 volt system to behave like a 12, Tim's story is a perfect example. If it's cold it will only perform worse.
Sorry to be on the bandwagon to change to 12 volt, but you did mention driving it in the cold. Yes, people drove 6 volt before, but all of the folks I know that were around when 6 volt was as delivered OE tend to carry jumper cables. How many people do you know now that don't even own a set?
I suggested the other things so you can drive it now, while you collect parts to do your 12V swap (large leads won't hurt a 12V system).
I'll toss 1 vote for an alternator. Not pretty, or 'period correct' but there's something to be said for being able to walk into just about any auto parts store in the land and even the new hire behind the counter can find it.
