How do I build my own LS1 swap harness for a third-gen, and can I use a non-LS harness or PCM as the base?

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

Harness and power

• Gen III PCM; Aeroflow AF49-1505; LS1/LS6; DBC; 4L60E/4L80E: AF49-1505 engine section retains injectors, coils, sensors and transmission wiring. (Aeroflow Performance)

Computer and tune

• 1997-98 LS1 PCM; LS1; 1997-1998: The 1997-1998 LS1 PCM carries service code 16238212, uses a single integrated computer board built around a Motorola 68000-series processor with 512 kilobytes of flash memory handling both time- and event-based computation. (GM High-Tech Performance / MotorTrend)
• 1999-2000 LS1 PCM; LS1; 1999-2000: The 1999-2000 LS1 PCM carries service code 09354896. Its connectors look like the 1997-98 units but the body is smaller and the pinouts are vastly different, so plugging a 1999 PCM into an older harness can damage the PCM. Motortrend states the 1999 unit carried over unchanged into 2000. (GM High-Tech Performance / MotorTrend)
• Sources differ: LSX Magazine's ECU guide states 2000 ECUs need 'some re-wiring' to retrofit, contradicting Motortrend's claim that 1999 carried over unchanged into 2000 under the same service code.

Confirm against the instructions for your exact parts before wiring. More: Harness and power in our tuning guide.