That story is basically a popular myth - about the only part of the Boeing C-5 proposal that made it into the 747 design was the JT9D engines. The rest of the 747 was basically a clean sheet - in fact what Juan Trip wanted wasn't a huge widebody, it was a double decker - basically two 707 fuselages stacked one on top of the other. It was Joe Sutter - as the 747 chief engineer - that came up with the idea making it a really wide single deck and managed to sell the concept to Trip.
At the time of the 747 development, Boeing was hot and heavy into the SST development (and in fact Sutter had to fight dearly for resources since most of the money and manpower was devoted to the SST )(same thing happened during the 747-8 program - most of resources went to the 787 - there was great satisfaction among the 747-8 team when we certified before the 787 despite the 787 getting most of the resources and a several year head-start). Further, as Asturias notes, it was believed the 747 would have a relatively short shelf life as a passenger aircraft as future long range travel would be by SST. Hence the 747 was provisioned to make a good freighter from day one (something Airbus failed to do with the A380).

From Air Pictorial May 1964:

https://cimg7.ibsrv.net/gimg/pprune....5848b03b95.jpg

BOAC bought a 707-379C G-AWHU off the shelf (already painted and built for Saturn but their order for 3 was NTU) to urgently replace the lost G-ARWE,
Likewise G-AYLT was the last 707 bought by BOAC to replace the SVC10 lost at Dawson's Field, Jordan.

As WHBM outlined >
The 707-320B has the range benefit over the -320C but many Legacy airlines bought both types.
The -320C at the time was chosen early on by a lot of airlines to also enable MAC work (US Troop movement charters)
The Vietnam War was in full swing - However the trend changed to ordering the DC-8 63CF series especially for US supplemental charter airlines.
Some airlines cancelled their 707C orders to buy the superior DC-8.

Trek Airways had ordered a new 707-350C from Boeing for its JNB-LUX route in association with Luxair, but the aircraft went from the production line to SAA to replace their brand new 707-344C 'Pretoria' lost at WDH in 1968.

..and if IRC that specific accident resulted in the 14 flap gate modification.

well that Air Pictorial goyt it a bit wrong - "likely to be the last long range, sub-sonic jet airliner in the Western World"...................... ;)

As I understand it, it wasn't just the range on Tokyo to Moscow itself that was marginal for a BOAC 707-320C. The diversions and service recovery allowed at Moscow were pretty poor, so I believe they always carried fuel for Helsinki, and in fact operationally preferred to go just a bit further still to Copenhagen if possible. At least one diversion made it as far as Amsterdam I once read - did anyone actually get right through to Heathrow ?

And the resale rationale was borne out to be justified, as many 707-320Cs went on to fly with a succession of cargo operators into the 1990s and early 2000s, while the passenger aircraft tended to be retired rather earlier. That said, the buying-up of JT-3D-engined 707s for the C-135 re-engining programme of the 1980s saw a large number of 707s retired when they must still have had plenty of life left on the airframe.