What signal source should feed the neutral safety switch input to the ECM on a 2002 LS1/4L60E swap into a 1975 C3?

Here is what the documentation says. Each point links to its source; details depend on your exact computer, harness and transmission.

Transmission control

• American Autowire's Classic Update instructions wire the neutral-safety switch in the starter circuit: purple wire 6 runs from one neutral-safety terminal to the starter solenoid 'S', purple wire 5 feeds the opposite neutral-safety terminal from the ignition switch, while light-green wire 24 is the backup-lamp output and pink wire 39B supplies ignition power for the backup lamps. (Api)
• American Autowire's update instructions state that with a factory manual transmission the purple #5 and #6 wires are connected together if no neutral-safety switch is used, where #5 is the ignition-switch start feed and #6 is the output to the starter solenoid. (Api)
• Lokar's shifter instructions (INS0239, via manuals.plus) require the neutral-safety switch to have continuity in Park and Neutral only, specify wire #12 or heavier, and say that if the fuse panel has no neutral-safety connections the purple ignition-to-starter wire must be cut and routed through the switch. (Manuals)

Other wiring

• P01/P59; Gen III LS; 4L60E/4L80E; 1999-2007: PSI standalone harnesses include an orange Park Neutral Signal wire that is explicitly NOT a neutral safety wire: it is a PCM strategy signal that goes to ground when the transmission is in Park or Neutral (used for idle/shift strategy), and must not be wired into the starter circuit. (PSI Conversion / Performance Systems Integration (via SlideShare))
• Sources differ: Contrasts with forum advice to delete the range switch entirely: removing the switch also removes the PCM's park/neutral strategy signal used for idle control.

Confirm against the instructions for your exact parts before wiring. More: Transmission control in our tuning guide.