Not all big block A-bodies had torque boxes. My 69 383-S of which I have the broadcast sheet, window sticker, fender tag, movie of the car when new, and have spoke to the original owner does not have torque boxes front or rear.
Rod
Just like yourself, One of the 5 dodge demon 340"s I own that is original and never been wrecked or repaired in any way, Built at the Hamtramck plant was built with NO torque box stiffner plates front and rear.
It is a 4 speed car to boot !!!
I have heard of other cars that also where high performance mopars that where also known to have Never Been damaged, Nor repaired that did not get them for some reason.
Yet, the other 4 demon 340's we own all have them front and rear! (two 4 speeds, two auto 's)
We have left it alone so far, But I am looking at buying some and adding them for piece of mind.
My other muscle mopars have them though....
It must have been quiting time when they built the car!