I had not realised the 763 had that much more volumetric capacity.However the 73F gross weight is fairly close to the 763 and the fuel capacity is nearly identical.
It is certainly a closer comparison than the the 75F and illustrates the efficiency of the (more) modern design.
Make no mistake, I am a great admirer of the DC8 in general and very impressed with its longevity, even before the CFM 56 retrofit it was a long range machine, with the new fans it rightfully gained an extended new lease on life with its stunning performance.
I wonder how many 707's would be around if it had the taller gear of the -8
allowing a similar retrofit, the USAF reengined their tankers but the civilian market just didnt seem to like it.