I've put a description of the method I used to cast the bed for my gingery lathe up on my website. I found it a lot easier than the method described in the book, so if you're thinking about building the lathe someday it might be worth giving it a try.
I was initially worried that the sand cores might float on the molten aluminium, but this wasn't a problem in practice. I just placed them on the drag and they stayed put.
-- Andrew, email@example.com