I can't answer these specific questions being asked but I can tel you that the airplane wasn't stretched.
The 71's are the same legnth as the 61's.
The 72's are the same legnth as the 62's.
The 73's are the same legnth as the 63's.
UPS doesnt have all of them either. They are everywhere.
Some operators include...
NASA Langley Research
And a few others.
The DC8s were converted by Cammacorp and were brought back to zero time airframes.
Airborne only has 50/60 series DC8's. No converted 70's.
There was no increased MGTOW increase even though thrust was substantially increased from 18,000- 24,000 pounds of thrust per engine.
This doesnt answer any of your questions but it may help.