As stated no need to switch anything the original two six volts in series make 12 volts so a single 12 volt will work just fine. Might consider going to a Group 31 battery with stud type connections and going with good heavy 2/0 welder style cables. Thread I made on my 4040 conversion plus the addition of a disconnect switch. Also gave cost run down of keeping original setup versus going to single 12 volt battery. https://talk.newagtalk.com/forums/thread-view.asp?tid=655904 | 
Converting from 2-6 volt to 1- 12 volt battery on JD 4630
