MTB frames tend to have freakishly long top-tubes when setup as a road bike. If you select a MTB frame just based on the top-tube length, then the seattube (and headtube) will be very low and require a longish seatpost and very hi-rise stem to get road bars to a comfortable touring position. Bottom Bracket height will also be fairly high in comparison to a purpose built touring bike. A sturdy steel MTB will be nice and stiff for handeling load, just make sure it is not pre-thrashed and ready to crack in half. 26" wheels can be more sturdy/stiff but will be slower if you are staying mostly on pavement.
A lightweight road frame can definitly be too flexible if you are carrying a load, the extra stiffness of a heavier frame is well worth the weight tradeoff.

If you are up for a big project, I suppose you might start with a steel MTB with appropriate seattube length, then cut out the headtube, shorten top tube and downtube (to get TT length to fit better) and then add a new taller headtube back in.
You can also change position of the cantilever bosses if you want to set it up for 700c wheels.