The approximate starting price for a custom LSx harness is about $650 (1999 - 2002 Gen III engines - LS1, LM7, L59, LQ4 & LR4).

The term "engine harness" could be confusing, maybe "chassis wiring" is a more accurate term? Typically, wiring to / from the LSx engine / LSx computer is considered "engine harness".

Integrating "chassis wiring" into the custom LSx harness is something that would only be done with new wire.

Perhaps a LSx engine harness kit is an option to integrate into existing chassis wiring as a DIY project? For example, a LSx engine harness kit could include 8' pigtails for the following:
ALT - Alternator
CMP - Cam Position Sensor
CKP - Crank Position Sensor
CTS - Coolant Temperature Sensor
Coils Packs
IAC - Idle Air Control Motor
IAT* - Intake Air Temperature Sensor
Injectors
KNK - Knock Sensors
MAF* - Mass Air Flow Sensor
MAP - Manifold Absolute Pressure Sensor
TPS - Throttle Position Sensor

* Available as a single 5 pin connector or separate 2 pin IAT connector / 3 pin MAF connector.

dave w