I wonder how many 707's would be around if it had the taller gear of the -8
allowing a similar retrofit, the USAF reengined their tankers but the civilian market just didnt seem to like it.
I suspect not all that many.
The early build quality of the 707 (especially the -320 straightpipe model) was not all that good, due to skin fatigue cracking issues.
Later models were better built.
I suspect the DC-8 was better still.