a proper touring bike will have a more upright position and be more comfortable (but not as fast) than a road bike. It'll also have a lower bottom bracket for stability and the frame geometry will also compensate for rear mounted racks/panniers so they don't catch your heels.
In addition there'll also be plenty of bosses for mounting bottle cages/front and rear racks/mudguards etc.
I bought one a while back - Kona Sutra - and I love it. Use it every day for commuting, been on a 3 day tour round scotland, couple of century plus rides and a fair bit of off-roading (forestry trails and the odd bit off singletrack and will be doing more now I've got some off road tyres for it.
IMO - Old MTB frames are OK for short touring, but some(not all) have the bottom bracket a lot higher than a tourer making the bike unstable at speed when fully loaded.Add to that the possibility that the chainstays will be shorter and could possibly mean you catch your heels on your panniers I'd drop the wedge and get a tourer.
Have a look at - Kona Sutra, Salsa Fargo, Surly LHT, Dawes Galaxy