What is needed to incorporate the LS harness into the stock wiring of an older truck?

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

• 0411 PCM (GM service #9354896 or 12200411); 1999-2007 Vortec DBC (LM4/LM7/LQ4/LQ9/L33/L59); T56 or non-electronic automatic: Five-path grounding scheme with 4-gauge straps. (PSI Conversion)
• Howell LS-1 standalone harness: Howell LS-1 harness grounds at the back of the LH head; CARB version keeps emissions. (MOTOR)
• Sources differ: Standard-form standalone harnesses (PSI, Aeroflow, BP) delete EGR, AIR, CCP and rear O2s; the CARB direct-replacement Howell retains all emissions functions.

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