Only right at the end of the 707's life at BOAC did they finally go for the developed 707-320B for passenger service. When they got the route through Moscow to Tokyo in 1970 they bought two such aircraft new specially for this. It was a strange purchase, coming after BOAC had taken delivery of their first 747s and as they were getting ready to dispose of the 707-420 fleet. Furthermore Tokyo to Moscow at Great Circle 4049 nm is nothing like London to LAX at 4741 nm.
The -320B gave a bit more range than the -320C as it didn't have the cargo door/strengthened floor structure; if they were that marginal that they couldn't use their existing -320Cs on the route then it must have been well beyond the old -420s.