oldstuff

Looks like a good Idea.
I would try for a pulley size that is about the same as the winch drum. If you go too small you risk damaging the cable by sharp bending over a small pulley.
Another thing to consider is that your snatchblock is probably going to be pretty close to the winch. This will result in the cable wanting to rewind in the same spot on the drum, possibly causing a big lump of cable in one spot. Something to think about.
Ray