Will the largest chainring on my road cranks (2 chainrings, 52/39 shimano tiagra cranks) sit further inboard than the existing (MTB) when fitted? If not it's well clear. My frame is quite narrow at the rear.
If it is going to sit more inboard would be good to know before I get a mech I won't be able to use...!
The road chainset won't sit anymore inboard than an mtb chainset. However larger road sized chain rings might get close to the chainstays of your frame, as they might kick out more than a road frame chainstays. Thus you might need to fit a longer BB axle to clear the chainset from the frame and in turn need a wider sweeping front mech.
It sounds you have it all in hand anyway.