Following was the reply from TAD (Trans Atlantic Diesels)...
Jack
Gentlemen,
I wish I had an easy remedy for your problem. Unfortunately the answer is tenacity, which I realize is not as readily available in a hot climate.Wooden wedges driven between the tank and the head is a good way to keep even pressure applied while you hope the penetrating fluid goes to work. Use a good quality product such as Liquid Wrench.
Don't forget to use anti seize compound when re assembling. Good Luck !
Regards,
Marcus Neville
Trans Atlantic Diesels, Inc.
... and, per Dave Kall...
"I got it out by cleaning out some of the gasket and cutting the bolt. Then used an easy out to extract the threaded stud. Now I just need to get the bolt out of the tank."
All's well that ends well! I'm sure glad that I'm sitting here in the Everglades working on my boat, rather than being stuck into Tahiti, like Dave ;-)