Add a switch to the wire going to the fuel pump. You do not want to be messing with anything related to the Engine Computer. For a number of years GM has purposely designed in a lot of tricks to cost unsuspecting owners money after the sale. Only BMW and Volkswagen are worse. Even simply disconnecting the battery on some models can lead to heartache and a very expensive tow and repair bill from the dealer.
Anything you do to the Engine Computer's circuitry will be detected as a defect, will set a diagnostic fault code, and turn on the Check Engine light. Unless they're doing something I'm not aware of, the fuel pump electrical circuit is not monitored so interrupting its current flow will just cause stalling within a few seconds. You could also add a switch in series with the ignition switch but be aware they almost always have three of four separate switches built into one assembly. Turning one part off without the others might set a code too. If you have the new push-button start novelty, a computer is added to the circuit to add even more unnecessary complexity. I haven't worked on any of those yet but I can't imagine anything good happening from modifying that.