Arthur,
These batteries are available in USA, through places like Macs Auto etc....
The catch is, they won't ship one through post, due to $$$ and acid. Obviously the acid issue can be overcome, but I looked into buying one for my truck some time ago, and could not find anyone interested enough to help. If you have any contacts who ship anything via container from US, then problem solved. There is also a couple of places in NZ that might have them.
http://www.earlyfordparts.co.nz and
http://www.fordv8parts.com Hope you have deep pockets. These batteries are hugely expensive.
Alternatively, you could use an empty vintage battery shell, and hide the real battery somewhere else. This is what I did. Using a correct carcass, I made the top plate from scratch, placed the dummy battery in the carrier, and then a modern battery will be hidden inside the tool box behind the cab. Cables will still go to the fake battery, in case I ever need a jump start. Much easier to connect jump leads to the exposed battery. One technicality, the battery inside tool box MUST be sealed unit or you might have issues with fumes in confined space.