Disclaimer: all the following is based on book reading. I respect Ron's experiences and advise. I will disagree with Ol' Ron about where the center of the combustion chamber is on our flathead V8s. He is correct about wanting the plug in the center of the combustion chamber so the flame front is as short as possible.
Consider the Ricardo combustion chamber. If you get the piston close enough to the head then the edge of the combustion chamber is effectively the chamber area cast into the cylinder head, not the far edge of the cylinder bore. The area between the piston and the head is so close that it doesn't play into the ignition event. This is discussed in detail by Mr. Ricardo himself in his book. I expect the Ford engineers who placed the plug originally read that book and followed his example along with some experimentation of their own. For anyone who want to read up on this, a pdf of the book is available for free here:
http://www.mediafire.com/download/gn...cardo+1931.pdf
There are two places in the book on spark plug location. They start respectively on page 45 (page 49 of the pdf) and page 98 in the book (page 103 of the pdf). I'd love to have an in depth discussion of these theories along with practical experience (the kind that Ol' Ron has way more of than me).