Well, the Super DC-8s (-60s and -70s) all began with the DC-8-61.
The -61 was simply a strech of the -50, and was intended for short, high capacity domestic hops.
Then came the -62, which featured only a slighty strech over the -50, new wings, and a much larger fuel capacity along with other things as it was intended for long range operations.
The -63 combined the -61's long fuselage (exactly 30ft longer than the -62) with the -62's wings to create a higher capacity (than the -62) and longer range (than the -62) aircraft.
The -70s are -60 aircraft (61 -> 71 and so on) which have been re-engined with CFM56 engines. These provide for longer ranged, quieter, and more fuel efficient aircraft.
In most cases, the DC-8-73 is the ultimate DC-8. It has a long range with high capacity, putting it in the same class as the A310-300HGW and B767-200ER. In a few other cases, the -72 is the ultimate, as it has an enormus range (If I remember correctly, it is equal to, if not greater than that of the A340-200, B747-400, B777-200ER, and MD-11ER!).