I too have understood that these 20-hour nonstop schedules from Europe to California actually featured an unadvertised refuelling stop, not shown in the timetable, at Winnipeg. The SAS DC-7C from Copenhagen to Seattle would stop at Sondrestrom in Greenland.
Regarding the SAS trans-polar flights which were pioneers on those routings, the problem that had to be overcome was that at certain times of the year, with no other radio navaids available, celestial navigation at high latitude at certain times of the year was not posible because the sun stayed just below the horizon for many hours, out of sight, but the sky was still bright enough that stars were not visible. There was a patented optical instrument developed which finally overcame this problem and allowed the flights to start, seemingly by allowing an accurate interpolation of the sun's position beneath the horizon. I bet close proximity to magnetic north also compounded any discrepancies. There was detail about it all in a Propliner article on the SAS DC-7C trans-polar operation some years ago, sorry I don't have the time to go through all the back issues.