Maybe.

To keep the axle from getting too long, the wheel is dished inwards on the right side to leave more room for the gears. If you look at your spokes, you'll notice that the ones on the right side are at a much steeper angle than the ones on the left. This means the freewheel threading is too far to the left relative to your chainrings, although with a 6 speed you should have a good chainline going from a single speed freewheel to the inner chainring without having to mess with spacers. An SS freewheel's cogs will be about where 2nd gear is on your current freewheel.

However, I would NOT file the dropouts to increase chain tension as suggested in the article.

You may also find this thread helpful:
http://www.bikeforums.net/showthread...bikes-to-SS-FG