Looking at my one, you need a special wire piece at the top to replace the current one which has two wires coming off the top loop rather than a single one leading down to the pulley and two pulleys that give you I think a 3:1 purchase and a hell of a lot of rope! (four times the height from the bottom of the mast to the shroud plate).

There's a picture of one at murrays http://www.murrays.com/archive/30.pdf in the bottom right hand corner