4322
Boeing 727/DC-8 are quite economical. Later on the Airbus 300 and 310 for very big routes.
Of other large aircraft the DC-8-60 series (the ones with 260 seats) are good, as are the MD-80 series.
In 1970 I would choose 727 or DC-8 (or some British aircraft, HS121 Trident I think) - really anything with 160-220 seats is good. If you like Soviet aircraft then the Ilyushin Il-18 and Tupolev 154 are good