If you can weld, get a nut the same thread as the petcock and weld it on.
Or, get a piece of solid steel rod, cut, drill and tap it, and weld it on.
I grind an angle all the way around the bottom edge of the nut so it has a nice round channel for the weld pool.
Oh, and drill the tank first, file the hole and get all the steel chunks out.
Pretty simple if you can weld.
(Obviously clean the tank out, don't blow yourself up, safety first, etc...)