Might also be the starter... I had a Studebaker lark that would barely turn over. Went through the entire electrical system trying to figure out why the battery didn't have the juice to crank it, new cables, batter, starter solenoid, etc. I finally pulled the starter, and found that it was so fill of dirt, crud, and dried grease that it wouldn't turn. The bushing in the nose of the starter was worn and sticky. Cleaned it out and lubed it up, and it cranked like a new car.
Also, for what it's worth, my Wagon with a 6-226 and 12v system (all new) still cranks very slowly when it has been left out in the cold. I think it's because of the heavier weight oil I have in the engine, which refuses to budge when its near zero.
Pete