The original 8 (especially the Rolls Royce Conway engines were 'gas'hogs,but on a cold day the conway could put out 20000lbs thrust)
The model 61,long fuselage,small 50's series wing was a bit of a dog-didn't like to go too high. The 63 came with P/W JT3d-3/7
Good for 19000lbs thrust,and the redeveloped wing,an excellent aircraft,until the 70's came with the CFM56's.
The P/W JT3D proved to be so 'popular' that with the grounding of the 707-300 series(noise/fuel costs) the USAF acquired the engines off the grounded aircraft for refit to the KC135,to extend their range.