You're right on keeping connections down to a minimum. 6-volt systems can have a voltage drop of 1/10 volt per connection, so adding a cutoff switch adds at least one more connection to the circuit, two if the switch isn't incorporated into the battery post terminal. Total voltage drop shouldn't be over 1 volt, adding all the connector losses and voltage drop in the cables. BIG cables are a must, to carry all the current possible and get it to where it needs to be- - - - -actually running the starter!
Jerry