I have old US DOT headlights in my 73. I would love to have HID lows and killer 100 watt high beams (with relays, of course), but what conversion kit do I get to house the two bulbs? (H...??) And will the HID bulb match the physical dimensions and light ray pattern the housings, reflectors and glass were designed for?

Any links or clues would be great!

I'm with Wil ... the HID cartridge isn't aligned with the lens of the H5 assembly ... so it is mostly producing glare, not a usable pattern for the high beam. Kludge, but with a nice touch ... the kit gives you four awful splice connectors that a 911 owner should be punished for using on his/her car's electrical system!

Not for H5, but I missed that part earlier and thought about something for H4, and found a thread with exact same idea: H1 clear lens and projectors

And if the clear lens is too boring, you could mount the projector close to the lens and sandblast the unused clear part, a la 993 litronic
http://departstop.com/images/993lit.jpg

here it is the Bi-Xenon upgrade
and this is the Xenon Low/halo HI