Hola beachbum -
xxx
I did not fly the DC-8 in the "cheap fuel era" prior to NOV 1973...
So all I know, it was flown at much lower Mach nš than the 707's ".84"...
Possibly at Mach .80, maybe .82, although I doubt it was that high.
xxx
When I flew the DC8-50/61, we cruised them at .78...
But the 62/63 could be flown at .80... better wing profile.
And the 71/73 with the CFMs, you want .82, go for it.
xxx
The "Stretched" DC8s were great passenger planes... 250 cabin seats.
Cargo 55F/61F had "just" ok payloads, 707C had same payload, better range.
Took the 63F/73F to have better payload than 707C... 20,000 lbs extra.
xxx
As you say, there are some of these DC8-63F/73F left operating.
Still great airplanes. But not many years left for them.
xxx
Happy contrails