[Bob Bidonde]

Stainless steel requires a "Passivation Processing" following any machining / welding to remove foreign metal that gets embedded in the stainless. If not Passivated, the machined / welded zones corrode & fail. I suspect that the failures of the stainless steel exhaust pipe welds are due to the lack of Passivation. Passivation is done to the machined / welded places with a weak acid bath.
In my opinion, the muffler clamp on the Model A is not structurally capable by itself of causing the tailpipe weld failure. However, if the clamp does not allow / restricts the motion due to heat expansion of the muffler assembly, it raises the stress in the pipe joints that hastens the failures of the corroded joints.