modifying MTB for road use
I've recently started using my Cannondale M300 commuting bike for road riding (got skinnier tires on it). Problem is that on flat ground I run out of high gears. So I would like to switch out the crankset to a road crank (was thinking of Sora or Tiagra). But the kids at two local shops have suggested that this is not possible, but I get the feeling they just don't know. Plus they want to sell me a new road bike instead.
The bike has 8-speed MTB cassette and chain that I'd like to keep if possible. So, would a road crank fit on Cannondale mountain frame and work with other components? Do I need to stick with a 8-speed crank or would 9-speed work as well? And will there be problems with chainline when going from MTB to road crank?
Any other suggestions/words of caution would be appreciated.