We can just note here that all the Boeing narrowbody aircraft, from the 707 in 1958 up through the 727, 737 (all models) and 757 along the way, have used the same fuselage cross-section, built to the same dimensions on the same tooling (if any has actually lasted 52 years), and apart from the 757 they have all had the same nose/flight deck exterior as well, leaving aside some details.
Yes the cabin interiors have changed many times (and are actually a customer variable option), but are ultimately fitted to the same structure.
Which I am sure is a record unsurpassed by any other group of designs. I presume the 747 cabin structure design comes second in such a design longevity consideration.All done in the days of drawing boards and pencils.