" In fact, the 747 was desgined with two "break points" where constant-cross section plugs could be inserted both in front of and behind the wing. I'm not sure if these align with what was ultimately done on the 748, but the point is a stretch is a relatively simple change."
All airplanes use the same basic concept to stretch - by adding plugs fore and aft of the wing. Engine location and engine weight changes dictate the length of fuselage plug sections; they may be unequal (typical of aft engine mounted airplanes) or fairly symmetrical (typical of wing-mounted twin or quad jets).
In the case of the MD
-90, only a forward plug was added, to account for the increased engine weight (V2525 vs JT8D-19 for MD
-80); in fact, the amount of stretch added to the forward plug necessitated movement of the forward overwing exit doors one window plug forward. That's how you can easily tell a MD
-90 from other Douglas twinjet models - the MD
-90 has two windows between the overwing exit doors.
The only criteria for a fuselage plug is that it occurs in a constant fuselage section; for ease of manufacture, chances are (I don't know for sure) that fuselage plugs were added at existing section breaks in the fuselage constructions, so existing sectional tooling don't need to be revised.