The DC-8 and DC-9 forward cockpit sections are pretty close. Initially, the windshield Part Numbers were different, but they have since been standardized so you can use the same unit on either airplane. The lower lobe of the two airplanes below the cusp is different and the fuselage aft of the DC-9 cockpit (the DC-8 cockpit is longer) is different. Right now, the only company flying both DC-8's and DC-9's that I know of is ABX Air, which is the airline subsidiary of Airborne Express.
In the "Oh by the way" department, the DC-8 was built with 3 different length fuselages; DC-8-10 series through DC-8-50 series, DC-8-61/-71 and DC-8-63/-63, and DC-8-62/-72.