According to my figures the 707-320C had a range with max payload of 4.300 mls ( 6.920 Kms ) and the 720B had a range with max payload of 4.110mls ( 6.450 kms ). So it's not just a big diference betwen both. About flying ATH-HOU in 707 non stop, that doesn't mean you have a bigger range, that means that the pilot really saved a lot of fuel flying in a great flight level and flying a very economical speed!!! You know, you could actually fly a CV990 from USA to Austria without making a tech-stop? Yes!!!! And the CV990 had a range of just 3.800 mls ( 6.115 kms )! The 720B had the same wing of a 707 with less payload, so that why it was used in special long routes where less passengers and less payload was needed. Boeing was very wise because they used the 720 to catch clients and letter offer them the longer 707, look at airlines like Ethiopian, AA, Saudia, El Al, Avianca, Aer Lingus, Northwest..... they started with the 720 and latter evoluted to the 707, brains, brains in Boeing!
CV990, the Maserati of the skies!