Update:

22 hours in... All rough cast is gone on everything that isn't under a timing cover or covered by flanges. Even the detail stuff is done. I'm finished wet-sanding with 220 grit. I estimate another 10-14 hours and it will be ready for the polisher. I hope I don't wake up with a blister on my thumb.

This is the coolant return side of the head. It's done with 320 grit now and you can start to see dull reflections of nearby objects in it. Things are moving right along now. I'm past the worst of it.

I'm going to use aluminum oxide polish as DanM suggested, after I finish with the 320, and hit it with 400, 600, 800 grit. I also have 1000, 1200 and 1600 grit, but that would be totally unnecessary on this piece.