How do I control electric fans on an LS swap without trusting the driver to flip a switch?

Automate it: a temperature trigger switches relays that power the fans.

• Triggers: a thermostatic switch in the coolant, a gauge or controller with a fan output, or your ECU's fan output (most aftermarket EFI has one).
• use a relay for each fan, triggered by a thermostatic switch or your ECU's fan output. Speedway warns that running two fans off one standard relay can blow its ~30 A fuse when both start at once; a single high-current relay works if space is tight. You can stage the second fan off the A/C compressor or a manual switch.
• Keep a manual override switch if you want emergency cooling, but the temperature trigger does the everyday work.