Install on the hot side as close to the battery as possible. You want to minimize the length of the hot lead between the battery and switch to limit the potential length of wire that can short when the switch is in the "off" position.
Not that anyone here is racing their old Fords, but NHRA rules require the master disconnect switch to be on the positive lead, as well as kill all electrical functions. The 2nd part requires some additional wiring beyond just sticking a big honkin' switch in the battery cable.