Hi Bob, Not specifically answering your question, but a friend of mine built a 31/2" Maisie back in the 1980's, as I recall there were no problems with the running gear, but when I was building the boiler there as degree of difficulty in getting heat for the silver solder to the front of the combustion chamber without melting the vertical water tubes.
In the end we opted for the alternative non- combustion chamber boiler.
Can't think of anything else.
Regards