Take a look at this sizing chart, for a mtb that you're going to ride on the road, you'll want one that's somewhere inbetween your ideal road and mountain sizes:


Compared to road bikes, mountain bikes have much shorter seat tubes (so that the back end of the bike can bounce underneath you as you go over rough ground with your weight on the pedals), and put the rider further behind the center of the cranks. If you buy a MTB with the same size seatpost as you'd want on a road bike, you'd end up with an extremely long reach to the bars, and if you buy a mountain bike that's the right size for trail riding you'd need an extremely long seatpost for efficient use on the road (and you'd end up with an extremely short top tube). So go for something that's between your ideal road/mtb sizes, you'll probably need a long zero-setback post and a tall stem too.

This thread in C&V is full of converted mountain bikes (my '84 Stumpjumper is in there) to look at for inspiration and guidance: Show Your Vintage MTB Drop Bar Conversions