You dont need the body loom. The only thing that crosses over is the black-orange and black-white (which comes from the black-orange wire anyway) or the main power source for the engine. The other is STA and IIRC you need two B+ wires. All these can be taken from the 20V ecu. The only other thing is the fuel pump which comes off the ECU anyway. Body loom i.e. lights, indicators, wipers, etc are all the same because you still use the same instrument cluster.
Other than that its just sensor input and injector/ignition output. Pretty simple.