I encountered similar situation on my '87 VW GTI 16V when wiring the kill switch. Alternator keeps the engine running quite fine when battery power is cut..... So I plan on interrupting the "T15" to the ignition because that will kill the spark which will in turn kill the fuel pump. Most vehicles should have some kind of fuel pump enable that's controlled by the ignition. So when the engine stops running the fuel pump stops pumping, no need for a separate fuel pump cut. This should also dissipate the charge from the alternator so there is no risk of blowing the regulator with a sudden shutoff of the alternator output.