Classic/Antique Car Repair: 1951 Dodge Wayfarer, 6 volt batteries, rubber isolator

Question
We are having problems with a 1951 Dodge Wayfarer. It cranks weak. We have had the starter worked on, but it hasn't helped. Someone told my dad that about a battery that would start up as a 12 volt, but then cut down to a 6 volt battery. Does that exist?  Would that help the problem? Do you have any soutions?

Answer
I have not seen one of those 12/6 volt batteries in a long time. However there is no reason that the engine should crank slow. I would start with the battery cables making sure that both ends are clean and bright. If the battery ends have been replaces by the repair terminals, I would replace them as they tend to corrode an cause high resistance. The make sure that there is a good heavy ground wire between the engine block and the frame, and then another one between the frame and the body. Remember that all of here parts are bolted together with rubber isolator to absorb vibration. Next have the battery load tested. If all is well and the starter is in good condition it should start well.