I think Esjaavik is saying "Yes, use old cut up cylinder heads". I've heard the words "high silicon bearing aluminum" in conjunction with sand castings. So if you use a sandcast aluminum for raw material, it should work again for what you want to do.
I found this list of shrinkages of various metals, under the heading "the shrink rule"
http://home.att.net/~philcannon/types.htm