Wow.
My first assignment at Boeing-Seattle was to conduct and evaluate high-alpha wind tunnel testing (at the large Boeing-Vertol tunnel near Philth-adelphia) of a proposed 727-400 featuring aft-mounted twin engines. And then to assess the flight performance.
In 1979!
Two candidate engines were examined: one was a derated PW2037 and the other, a GE CF6 variant. They had slightly different nacelle geometries hence the reason to assess the corresponding installation effects on aerodynamic drag and stability & control.
The 727-400 configuration had a stretched fuselage, but if memory serves, the stretch was different than the earlier 727-300 design concept, due to unique balancing requirements for the twin-engine arrangement, which included the removal of the 727 center intake, S-duct, and exhaust.
As far as I recall, Boeing had not yet addressed if the 727-400 would be offered as a retrofit change or only new production.
This 1979 Boeing project has not been discussed publicly as far as I can tell. The "727-400" concept mentioned on Wikipedia occurred a few years prior as the 7N7 program was evolving, and was a wing-mounted twin-engine configuration.
While I was working on the 727-400 preliminary design, full-scale 757 development was underway (in a nearby building at the Renton plant) following the launch orders from British Airways and Eastern Airlines in August 1978. Our team's analysis found that the operating cost per seat mile of the 727-400 was quite close to the 757, with a unit cost substantially less.
Not surprisingly, I was told that airlines were showing significant interest in the 727-400 instead of the 757, which Boeing had already committed big $$$ to develop.
The obvious question is why were Boeing's marketing folks showing the 727-400 concept to airlines in 1979? My guess is that the 727-400 was Plan B, if Boeing decided to cancel 757 development. Orders for the 757 had stalled -- the next major order would not be until November 1980 by Delta Airlines.
https://www.yesterdaysairlines.com/airline-history-blog/the-757-story-pt2-engines-sales
Sometime in 1980, Boeing management told my supervisor to stop work on the 727-400 activity. We worker-bees assumed the reason was that our effort was a direct threat to the 757, but that begs the question "why did Boeing leadership sponsor our project in the first place"? Cold feet due to bad 7N7 market analysis? Or did Boeing execs over-commit financially, with the concurrent development of the 757 down in Renton and the 767 up in Everett?
From reading the current postings, it looks like Pratt & Whitney approached Convair-San Diego (and maybe others like Douglas-Long Beach or Lockheed-Burbank?) about the same time Boeing halted its work. Fascinating.