The most difficult part of the job will be getting it square. If you are good with a hacksaw and file, you shouldn't have any problem. Get it as square as you can, then use the round headed brass screw amd valve grinding compound in an electric drill for the crown.
As far as cutting it down? I don't know the legal length, but it is somewhere around 16 or 17 inches. Make sure before you cut off too much.
