What wire to use from ignition to kill?

OK,,,,KISS method....

At the ignition switch connector. black w/ white stripe wire. Go back a few inches into the harness so you have enough wire to work with and cut the black/white wire. Install a spst (single pole single throw switch. This will kill power to the ignition (spark) circuit. You may need to splice on some xtra wire to get the switch hidden up under the dash a bit, or to run the switch to where ever you want to hide it.

Madeline and monkeyman...

On every year that I have seen from 70-83, either at the coil or at the ignition switch, black/white stripe wire is 'switched ignition' power. In other words when the key is turned to 'on' or to 'start' there is 12v present on that wire. So all you need to do is to install a switch that would in effect open and close that wire. On f/i equipped models it also provides switched ignition power to the f/i relay so (without pouring over schematics) it should also prevent the fuel pump relays from operating. I don't know how many amps it pulls but I doubt if it is much more than 3 to 5 amps. I wouldn't use a mini switch but a standard spst $1.98 style switch from Radio Shack should do the trick. Obviously there are much more elaborate and interesting ways to accomplish this but if you just want a simple 'kill' switch........