if its a pre facelift model I would recommend doing the GTA mod on the lights before you fit HIDs. this involves removing the lights, taking them apart to paint the chrome surround black and then putting it all back together.
when I HID'd my twin spark and later my GTA (spare set of standard light units) I removed the dazzle caps to get the bulbs in whilst I had the lights apart. passed MOTs fine like this.
as you already know, standard H7 HID bulbs are far too long for the caps and don't fit. I did try H7Rs in the standard GTA light, these didn't fit either.
now you can get H7C which I think David C may have fitted to his 156 although I've not tried them yet these are supposed to be the same length as a standard H7 filament lamp so should fit no problem. I believe David C said these fit no problem and I wouldn't doubt the mans word!
1999 156 1.8TS Sport Pack 3
2001 166 3.0 V6 24V Super
2003 156 GTA 3.2 V6 24V
2005 156 1.9 JTDM-Jet Sport
2008 159 1.9 JTDM Lusso
1997 GTV 2.0TS Lusso
2001 147 1.6TS Lusso