Ummm, it's the reach or control of the front end. I would think a no neck stem, maybe you can fit,...but really IMO, with mtb, it's better to over power the frame by riding SLIGHTLY smaller bike frames than say how you'll fit road.
My mtb has bmx 'ideas' going for it, but running a 16 seat, 20 toptube, when I fit a 17-18 is o.k. Harder as I had to custom, but control is super.
Because mtbing can be agressive, the ability to really muscle the front end is important.
Over extending your shoulders etc will make it very hard to pull the front wheel up.
If you have to keep it, shorten the stem, If an inclined frame, this should pose no problem.
a horizontal level toptube will make the seat position change a bit if shorting the stem.
IMO, yadda,yadda.
>jef.