Battery cut-off switch under the seat (but easily reachable). I have seen one used successfully in a 6v 1953 GMC. I do not know if the hot lead or the ground lead went through the disconnect.

http://www.elecdirect.com/catalog/4980ccb6-469f-4309-a3e9-bd8b08055bcd.aspx

http://www.harborfreight.com/heavy-duty-battery-cutoff-switch-66789.html