re the dc 8 super 60 series
the 61 was the longest ever airliner stretch up until then
basically a 50 series with a super stretch
the 62 was a small stretch over the 50 with many upgrades to wing and engines/nacelles with a HGW
the 63 was the 61 fuselage with all the 62 enchantments and HGW's
the 70 series was to Re engine any of the super 60 series with CFM56
the 61 series being a big mod to the wing/pylon area