Freewheels are not all the same width. 5, 6, 7 are all different. There are no mtb or road FW's. . . just different brands.

A 6 speed FW does not require as much room as the 7 speed. Do the math. http://sheldonbrown.com/k7.html Add up the cog and spacer thickness between a 6 and 7.

You can respace the hub and redish the wheel.