[dcb]

FYI, did a saturn s-series kill switch by putting a Normally Closed microswitch on the shifter in series with the "purple" crankshaft sensor wire by the ECU connection.

It's perfect, no codes, ignition and fuel both shut off when it is held in (even when engine braking) so it doesn't stumble on at all, and doesn't seem to have any strange side effects like the FI fuse rig did.

So far I've had good results with:
metro: interrupt cam signal
saturn: interrupt crankshaft signal

Difference being the metro has a distributor.

Of course you can use relays to interrupt both fuel and ignition but that complicates things and you would have two or more "switches" that can fail instead of one.