I know of the Salsa Cutthroat but was considering modifying or building up a 29er MTB with drop bars. Anyone have experience, recommended, pictures, build specs, etc. that they are willing to share?
100% agreed on the top tube comment. Mountain Bikes designed for flat bars have much longer top tubes than bikes designed for drop bars.
The reason being is that the hoods put you roughly 3 inches ahead of the stem bar bore. You typically spend a lot of time riding on the hoods, thus the TT needs to be short enough in order to not make you stretched out too much.
Think about it a typical 56cm drop bar bike usually has an effective TT length of around 560mm. Compare this to a 19" XC hardtail and you'll find something in the neighborhood of 620mm.
That's a 60mm difference, so if you put drop bars on that XC hardtail your cockpit combination will grow an equal amount, and 60mm in cockpit combination is a lot.
You can remedy some of this by running an extremely short stem, but it may not handle great if go this route.