Oh also, do a search for French flatheads… Apparently France exactly copied the flathead for use in their military trucks. As far as I know, they were still building BRAND NEW flatheads into the late '90’s. I guess a surplus of brand new blocks still exists, so that might be another avenue to explore.
Check this out too: http://www.hotrod.com/techarticles/engine/hdrp_0511_ford_flathead_engine/index.html
It’s the “Ford Flathead Guide of Death” from HotRod magazine. You seem to be knowledgable already, but it may help. It’s a 6 page writeup and the entire last page is just flathead parts sources. Good luck with it!