Since this is a 45 degree riser stem, the longer extension ones will raise your bars a bit but also move them further away from you. If the frame is too small, this may be just the trick. If the frame fits already with flat bars, then you probably want the shortest extension one.

Also, measure the extension on your current stem. Picture where it would place these bars, and then picture how it would be different with the Nashbar stem.

Trekking bars are good for turning MTBs into tourers. They give you at least three different hand positions to move between, and take MTB controls easily.

I put Wald cruiser 872 bars on my old MTB. They bring me more upright for around town riding, with lots of reach back. They lack the forward ("drops") position that the trekking bar give you, though. Personally, I'd try wide flared drop bars (midge/dirt drops) if I were to try touring on it.