Jonathan,
I cant see how the nut will travel the whole length of the screw either. I think your idea of moving the "pulley" end bearing a bit further in towards the floating end and you can machine the plain section down for him to accept the pulley. I dont think lengthening the shaft would be agood idea particularly if you are going to fit a pulley on the lengthened section. Without some kind of support this will start to wear and wobble.

Ian