Not sure what you mean, but aircraft stopped being "chassis and body" some time in the 1930's. That was the switch to a monocoque design with stressed skin.
I was hoping that the person I commented to could point us at some fun manuals to describe how these configurations worked at a technical level.
Doing things, however, that increased the weight or changed aerodynamics of the airplane were a very big deal, causing a ripple effect that would be very expensive.
Freighter versions were commonplace, with the obvious omission of windows (weight savings) and interior fluff.
Doing a stretch, or a re-wing or re-engine is an enormous thing.