From what I understand, the 98-00 LS400s were offered with two headlight options--halogen and HID.
The lens are the same for both the halogens and HIDs.
The outer lens (which even when made without a visible pattern/fluting, but instead looks 100% clear,
can have optical properties optimized for the headlamp it goes with) may appear the same, but are not identical. It's especially true of this vehicle; the pattern inside the lens
is different, but it doesn't end there.
The reflectors inside the lamps are different by as well: The halogen headlamp uses the HB4 (9006) but the HID headlamp uses the D2R. The shape of the reflector is different to accommodate the different shapes of the actual light source (filament vs. arc). There is no adjustment, no shimming of the bulb, no angling of the bulb, no reaiming of the headlamp, that will make a D2R work in a lamp assembly designed for the HB4; the reverse is ALSO true.
The
only legitimate way to put HID in your LS400 is to get genuine Lexus (they may actually be branded Toyota, like lamps on the '99 RX300) headlamp assemblies. (You could also buy complete Hella 90mm lamp assemblies but those would look a little odd sitting where the factory headlamp used to be.)
I've gotten excellent pricing from
Lexus of Melbourne (they're in Florida, not Australia) when my friend needed sidemarker lamps for his '99 RX300. On that vehicle, the exterior glass and all the lamp assemblies are branded Toyota, not Lexus, so don't be surprised if the part you order says Lexus on the outer box but Toyota on the lens.