I'm pretty sure it would work. Part numbers are different in the parts catalog, however when looking over the wiring diagrams and two harnesses I have here, they are identical. (98 & 99 GS400 harness)
Do you have a work around for the imobilizer? You'll need a key or send the ECU in to have the prior programming removed.
If you need an overall wiring diagram for the GS400, I have it in pdf form. Drop me a PM if you need it.
-B