I've been thinking about using a steel shell with a copper firebox and flues for a loco boiler with a 5.5" diameter. I know this was not an uncommon practice in full scale especially in English locos and traction engines. Seems like one would have conductivity where it is most needed and less...
The folks from Chesapeake and Allegeny Live Steamers have some useful stuff as well. Links to follow
http://www.calslivesteam.org/calboil1.htm
http://www.calslivesteam.org/calculations/Calculations.htm
Jim