Based on a lot of years as Pax the stretched DC8 61/71 and 63/73s had a really pronounced left fight and up and down moments being very long and thin the extensions being both in front and aft of the wing. Definitely the bounciest aircraft I have ever been in. On the other hand the -62/72 was a beautifully proportioned aircraft an seemed much much steadier in the cruise-but then i might have had my trips ona good day. 757-300 was about as close to the long DC8s you could get and they were a bit bendy at the back . Of course if you spent the whole flight looking backwards the MD 80s seemed to bend a bit as they really were long and narrow but in front of the wing. Before my time but i wonder what the Lockheed 1649 Starliner was like.