KC-135:
The KC-135 is actually a Boeing 717, this is what was EXACTLY printed on the dataplate of the aircraft, at least when I was in the USAF, I doubt that they changed it. So when Boeing updated and re-marketed the MD-80 and called it a 717 they were actually using a designation that had already been utilized, albeit not well known. FYI, from my understanding the new 717 (updated MD-80) is a fantastic aircraft but not well accepted by the industry.
From my understanding the KC-135 has more in common with the 367-80, than with the 707. The 135 does not have an Engineer's station, the 707/720 does, the fuselage cross section is I believe the same as the Dash 80, the 707 is substantially wider.
Another interesting tidbit that I heard was that there was 90% parts compatibility between the 707 airframe and the 727!
From everything that I heard through the years the reason that the Diesel Eight is still in wide service and the 707 is not is because of a number of different reasons:
707 was more difficult & expensive to hush kit or re-engine.
There were significant corrosion problems in the fuel tanks and the wing root area.
The airframe of the 707 was specifically life limited, while the DC-8 is based on condition.
General efficiency. I remember a maintenance guy telling me a story about being out in South America or Florida. On the ramp was a 707 & a DC-8. He was fueling them for a flight to the same destination. He said that the 707 had 13 freight positions and the -8 had 18. He loaded 120,000# of fuel on the 707 and 95,000# on the -8.
I guess that would about sum it up.
This all of course is just what I have heard, by it all sounds plausible.
Emery conducted substantial overseas operations with their -8s.
The 70 series Diesel Eights are incredible machines, the -73's (longer wings) were my favorite. They fly like trucks and you really have to be careful in planning your descent as they do not like to come down. With no speed brakes you would either have to use the landing gear or if absolutely necessary you could pull # 2 & 3 engines into reverse inflight & past the interlocks! Pretty impressive, though considering the age of the aircraft and that there were pylon cracks developing it is not a good practice.