When I get home from a Mopar show my knees are usually dirty from looking at the bottoms of cars. I've been unofficially trying to come up with the rules when torque boxes were applied. Automatics generally are less brutal on drivetrains. (at least when these cars were built) Convertibles generally got extra reinforcements also, not just Chrysler but Ford and GM too.
My 69 'cuda340 has front AND rear torque boxes. And I'll throw into the mix that the axle ratio may have influenced the rules. This one has factory 3.91 gears and a 4 speed. The driveshaft is HD and the front sway bar is 15/16", not the typical 7/8". For some reason this car was built with extra heavy duty parts.
I do a lot of reading about the Chrysler tradition and have always been impressed that the Chrysler engineers pretty much did the right thing. (we had forged cranks in our lowly base 318's for instance) GM on the other hand always counted their pennies. I have bought and read probably 6-8 books on the Chrysler heritage, WPC himself, the 3 Musketeers of Chrysler, We were the Ramchargers, etc. trying to learn why they did these things. In my opinion, if the engineers thought these cars should have had sub-frame connectors, they would have put them on. Screw the cost. They were really the first to apply computers to analyze data and today's FEA programs can show all kinds of suspension and frame weaknesses. I'll take our unibodies over full-framed GM and Ford cars. 67-69 Camaros with bolt-on sub-frames? Really? You think that's stronger and less flexible than our junk?