First off figure out if you have a 68 or 72 mm BB shell on the frame. Also the chain stay mount at the front is often different between MTB's and road bikes thanks to the widely different width of tires. So it's quite possible that when you mount the road crankset to your present BB that the clearance between the crank arms and the chain stays will be just right. I know that switching road for MTB cranksets to get to what I want I've only had to switch out my BB sometimes due to the shift in the chainstay clearance situation. So it's worth getting the CRANKSET, not "chainset" first and try it with your present BB. If it doesn't work out then BB's with different axle lengths are pretty easy to get from your LBS's so the time frame penalty isn't that big a deal.

Also due to the larger chainring sizes of a road crankset and the wider positioning of the chainstays on a MTB you may NEED the longer stock BB to ensure that the rings do not interfere with the stays. Again a very good reason to not jump into buying a new BB to go with your new crankset until you know you need one.