Front conversion will be fine, need to drill out the 2 holes on each side, from 10mm to 12mm to take the caliper bolts...

need washers/spacers too on the front about 14-15mm thick, if i remember correctly...

no one, has done a ST170 rear brake conversion yet, not sure at all about this tbh!

people have done normal focus rear disc conversion though! do you have the rear conversion plates to mount the caliper on to ?