I have just had a look at the HID's on the ebay site following your link. These lamp units are built into a set of pop up lamp covers. I'm sure some chap associated with the S12 club had built a set of these. I have a sneaky feeling he may be directly or indirectly involved with these on the ebay site, my thinking here is that these units would fit the silvia with minimal adjustment.
If you decide to go with these on the strength of this let me know how you get on.
Best regards,
john
