you need hope mount HBSM20 to fit an is caliper to a post mount fork but it will bump your rotor size up by 20mm, so if you're running 203 (which i think all 6ti's were anyway) on the front you'll have to find a 223mm rotor http://www.chainreactioncycles.com/Models.aspx?ModelID=13735
I've got the same problem and rather than start a new thread, I was wondering if:
a) Anyone makes an adapter for this that doesn't increase the rotor size?
and
b) Failing that, will the use of a 223mm disc damage the fork in any way? (I'm fairly sure it will mean the end of any warranty, but is the extra stress from the larger disc damaging?)