I agree with Mike. Make sure you have big heavy battery cables for the 6 volt system. Ask for them at you auto parts store or farm supply. Also I connect the battery ground directly to one of the starter mounting bolts.
what every one is saying is to use double ought,,,or 2zero or "00" sized battery cables and make sure of a cleaned ground contact metal to metal....then 6 volts should be good enough...especially with a rebuilt starter with good clean posts....
12 volt systems use thinner battery cables and do not conduct current well enough for 6 volt systems
Do You own a car built by Durant? 1929 MODEL 70 COUPE & 1930 6-14 SEDAN