Hi Malcolm
You might want to touch base with the Wright Museum in Wolfboro, NH
http://www.wrightmuseum.org/. They have had the misfortune of blowing the radial in their Sherman twice. They may be able to provide some information and what to avoid. They have a complete Ford GAA they could put in but want to keep the correct radial. In there search for parts maybe they have come across the parts your engine needs.
Cheers Phil