An inch thick or more is fine for the block, Witzend. Brass is more traditional than aluminium. If you do use aluminium be careful it is not cold-formed or it could be much harder than you expect.
Putting the nut onto the shaft is better than just hitting the end of the shaft with a hammer, but it is quite rough on the thread and in my view, is not really good enough if it looks as if you might have a slightly stuck shaft. If you need a proper hammer, you need a brass drift.
