Also, don't rule out curricula that has been released in the years since the 2nd edition TWTM was released. Two that come to mind are JAG and Growing With Grammar. I am using Growing with Grammar with my third child and loving it.
Right now I am using Usborne's First Encyclopedia of the Human Body. It was not published yet when the last TWTM was written. I think it is an excellent spine because you can download pictures to label and they have found lots of cool websites to expand on the topic.