So where we would find the common point is the Wiki range of 3760nm is a ferry range with no reserve = AWS ferry range of 3420nm with reserve of 340nm.
Still seems strange that this aircraft would even be constructed, since it is useless for transatlantic flights and limited (to about 100 pax) on transcontinental flights)
Keep in mind that the planes in the 1960s-1970s had economy seat pitch of something in the range of 35-38 inches and there were first and economy class lounges (along with a roast the "stewardess" carved at you seat

. It would not at all be unrealistic if an early D.C.-8 only had 100 seats because it also cost the equivalent of about $3,000 in today's $ to fly across the country in economy.
I've noted before, but the other thing on the D.C.-8s is that the variants 10-50 were not new builds rather options achieved via switching to new and better engines that were rapidly developed in the early 1960s. I think the -60s were first variant with a stretched frame.
Not sure this link works, but if not bing Dc-8 seating charts. This united plane had said lounges and if I counted right 52F and 48Y
http://www.aviationexplorer.com/vintage_classic_airliners/United%20DC8.jpgThis is a decent site with pictures. Doesn't help with the range question, but does cover the fact that different variants were the same frame with different engines. So in essence those first-10s weren't junked after 5 years, rather they were re-engined and flown as -50s into the 1970s.
http://www.aviationexplorer.com/dc-8_facts.htm