Koncretekid posted the most comprehensive analysis of the forces involved, thanks.
Just a thought: if the purpose of the change is to improve the bending resistance (protects against intrusion, or folding the chassis) of the 1.25" × .090" tube by enlarging the OD to 1.625" × .120", that almost triples (291%) the current level.
Where's the math says that 291% is safe? Or needed at all?
An intermediate safety level (+100%) could be obtained in some existing chassis by simply adding another 1.25" OD tube adjacent and tangent to the existing - no larger skin, no less driver room, just more work and weight.
I'll assume based on unresponsive comments made on behalf of the legal department (who probably contain fewer engineers than this thread), try submitting this proposition to your liability insurance carrier, and see if you don't get a Fex-Ex back "CANCELLED".
Without getting all legalese, the concept is "reasonable assumption of risk", where if you require nothing and the car explodes - too bad.
OTOH, if you told him exactly what to do, and it doesn't work, you eat the whole "wrongful death", "loss of consortium", "projection of future earnings", "pain and suffering", etc.