Just finished my third MTB to roadbar-conversion today. First one is a nice old Kuwahara turned itnto a touring bike. Found out that a 19 inch frame is a tad to small for such a converson, the bars do not get up high enough. Second bike is a 20 inch Miyata turned into a studded singlespeed. Works great. Last one is a huge 22 inch old bonded Merida aluminum with the first 7-speed xt generation (decent bio-pace, actually works).
For the geared bikes I simply grind the old thumbshifters inside and force them on the end of the roadbars.Both have under chainstay U-brakes and it is a bit tricky to set them up but they work OK.
Seems to me that this sort of conversion works best with a large frame.
I enjoy passing 20 year younger guys on carbon MTB wonders costing 40 times my fleamarket bikes on the fireroads.