Which standalone harness company should a first-timer use for a 5.3 DBW / 4L60E swap in a 67 stepside?

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

Throttle (drive-by-wire)

• Holley lists separate DBW harness part numbers: 558-406 for the standard drive-by-wire harness and 558-429 for the early-truck drive-by-wire harness. (Holley)
• 242; LS1, LM7; 4L60E/4L80E; 1999-2007: Current Performance's Stocker Series 24x blue/green electronic-throttle harness includes an ECM connection (blue and green), an eight-pin black throttle body plug, a sixteen-pin gray TAC module connection near the ECM, a pedal sub-harness (ten-pin black to ten-pin gray), a two-pin VSS plug, and an optional round gray 4L60E/4L80E transmission connector. (Current Performance Wiring)
• 0411, 242; 4L60E/4L75E/4L80E; 1999-2007: PSI's Gen III 24x DBW controller kit for 1999-2007 DBW truck engines with 4L60E/4L75E/4L80E includes the engine/transmission harness, MAF sensor ACC-1015, two O2 sensors ACC-1002, accelerator pedal ACC-1049, TAC module ACC-1011, TAC module harness HAR-1022-C10, brake switch KIT-1002, and a tuned PCM PCM-1005 with bracket ACC-1087. (PSI Conversion)
• 0411, 242; 1999-2007: PSI's base tune in its Gen III DBW controller kit removes VATS, bypasses VVT where applicable, and disables DOD where applicable; any camshaft change, performance modification, or component differing from the order options requires a custom tune to avoid no-start, limp mode, or no throttle response. (PSI Conversion)
• 0411, 242; 1999-2007: PSI's Gen III 24x DBW controller kit is a standalone system needing only power and ground sources, a fuel pump able to support 58 psi (400 kPa) at the fuel rail, and an electric cooling fan; the harness is plug-and-play with a four-wire hookup (switched power, constant power, and grounds). (PSI Conversion)
• E40; LS2; 4L60E/manual; 2005: Howell offers DBW harnesses by application: HY5TD for a 2005 LS2 6.0L Corvette with electronic 4L60E and drive-by-wire, HY5T for the same combination with mechanical throttle, HY5D for a 2005 LS2 with manual transmission and drive-by-wire, and HY5 for the manual with mechanical throttle. (Howell Engine Developments)
• 0411; LS1; 4L60E/manual; 1997-2004: Howell also builds DBW harnesses for earlier Corvettes: HY92T for 1999-2004 LS1 Corvettes with electronic 4L60E and drive-by-wire, and HY78 for 1997-1998 LS1 Corvettes with manual transmission and drive-by-wire. (Howell Engine Developments)
• 0411, 242; 1999-2007: PSI describes its Gen III 24x DBW controller kit as a standalone system requiring only a four-wire vehicle hookup (switched power, constant power, and grounds), a fuel pump able to support 58 psi (400 kPa) at the fuel rail, and an electric cooling fan. (PSI Conversion)

Harness and power

• Gen III DBW: blue/green P59 PCM; Gen III DBC: red/blue 0411 PCM; 1999-2007 Vortec; 4L60E/4L80E: BP Automotive hookup wires: fuel pump, ignition, TCC brake, MIL, speedo, tach, fans, cruise. (BP Automotive)

Confirm against the instructions for your exact parts before wiring. More: Throttle (drive-by-wire) in our tuning guide.