It shouldn't be a problem. The biggest issues are the stem/handlebar fit and the brakes. OS stems/bars should match up well, regardless of whether the stem is road or mtn. Get the shortest stem with the most rise you can, as the reach in the bar will put you several cm forward on the bike. You may want to run disc brakes - get the Avid BB7 Road version and throw on a Travel Agent for a little extra cable pull. STI 9 spd brifters will work with Shimano mtn derailleurs, as the cable pull is the same. See if you can find some used Ultegra models for triple cranks.