The cracking was due to "Intergranular penetration of silver solder due to cold working and thermal expansion stresses".
This could have been avoided had I known about this single non-desirable property of the copper-nickel material.
It NEEDS TO BE ANNEALED, not only before, but also AFTER it is cold worked - such as bending out the sides of the firebox.
50 hours of work down the drain. The 2-6-0 project is in a state of hiatus at the moment, while I'm contemplating how to continue... :-(
For a thorough description of the boiler and of the reasons for the disaster, please see the extensive article in "Live Steam & Outdoor Railroading" magazine, May-August issues 2006.
Here's a good source of info, a 52-page tutorial on joining nickel alloys:
http://www.specialmetalswelding.com/publica/joining.pdf