I have no idea how he would have worked.
I know as me being a "junior structural engineer", nothing I did got done without a two step oversight of approval.
In the '70s, "modelling" was being used. It was before computers and still the age of slide rules.
As I said, I don't know where 400lb-ft comes from or is necessary to begin with. I will add that the rear upright of the Pantera resembles that of the Ford GT. That one being cast in magnesium though.
Was that modelling? Why reinvent the wheel. Take one off the shelf and plug it in.
I think the issue with stiffness of the "chassis" is just an under estimate of horsepower and tire gripping capabilities using 1968 standards.
The Mustangs, Cobras and GT40s all show similar indications of lacking here and there. Race cars were stiffened with roll bars and cages AND NO manufacturer (including Ferrari ) provided for their cars to still be around over 7 years. That number is still used and proven by the actuaries the makers use.
150 sounds like a guess? Got the formula? 