Re the might deisel 8, Super 80 series
there were three models
61, 62, 63
The 61 was for US domestic and had 4 CFM 56s replacing the old coal burners
the 62 was a lesser stretch by some way and had an extended wing as well as the CFM56s and had a very long range for the day -Moscow-Tokyo, LHR-BKK
the 63 had the long fuselage and bigger wing so it had the capcity increase but not the extreme range-it could still do TATL easily enough though
Although not a stellar success I think they were certainly not a stretch too far and both the long and shorter bodies were nice looking aircraft, a lot of both types ended their days as freighters and were very successful until yet another fuel price crisis caught up with them.
Stretches too far in my mind are
737 900, commercial success but apparently a dog to fly and needs very long runway compared tot he 757 it often replaced
Md 90 which really does look like a sort of civilian F104 , all body and no wings
737-300, if you start out with a long single aisle fuselage then stretching it doesnt do much for looks or versatile performance
And for the future-|I cannot see myself up the back on a 777-9X or whatever it is called this week