John,
with regards to the Boiler dimension / Wheel dimension, on the LBSC drawings for his 2-4-0 Web Compound "Jeanie Deans", there is a note to dent the boiler locally to clear the Driving Wheels. I don't know if this helps you as your boiler is lower into the Wheels. But may be considered. I would reason if the boiler passes a hydraulic test after the disruption (especially if it were made from drawn tubing, even better if done before silver soldering), I wouldn't think it would be unsafe.
As I near completion of LBSC's Netta (0-8-0), I would really like to embark on a 4-2-2 Stirling in 3-1/2" gauge. But it appears there are no wheels out there ??
Bruce