How do I reuse the stock GM underhood fuse block to make a truck harness standalone for a swap?

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

• E38 ECM; Chevrolet Performance NA LS control kit 19171935; LS2/LS3/LS376/LS7/LSX454; 58x/4x; 2006+: Fuse/relay center holds engine fuses plus spare positions. (Chevrolet Performance)
• E38/E67 ECM; Speartech 58x manual-transmission harness: Current Speartech fuse order: battery, injection/ignition, O2 heaters, fuel pump. (Speartech)
• Sources differ: Older Speartech LS-58X notes revision hosted at documents.holley.com: #1 fuel pump (orange), #2 battery (orange), #3 O2 heaters (brown), #4 ignition/injection (pink).
• E38/E67 ECM; Speartech 58X with 4L60/4L70/4L80 transmission: Older Speartech revision lists a different fuse order. (Speartech (via Holley))
• Sources differ: Current Speartech 58xman/58xA4 fuse order: #1 battery (orange), #2 injection and ignition (pink), #4 fuel pump (pink).
• Gen III DBW: blue/green P59 PCM; Gen III DBC: red/blue 0411 PCM; 1999-2007 Vortec; 4L60E/4L80E: BP Automotive includes fuse/relay box, pump relay, fan controls, CEL, OBD2. (BP Automotive)
• 0411/P59 factory harness strip-down: DIY fuse-box rework: removed circuits are unfused or re-fused, then re-loomed. (LSX Magazine)

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