Robert, from what I can see your problem might not be down to the pulley and it's crown, but to flexing in the main collumn. I would extend at least the top shaft, and fit an outboard bearing. I would make the jocky pulley push / pull the belt in rather than out, this increases the belt wrap around, also make sure that the jocky wheel runs square, or it will throw the belt off. If all else fails, put a little rim around the edge of the pulleys/maybe just a large washer on each side, it just needs a lip a bit thicker than the belt.
Ian S C