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there really isn't a "cargo box" on a C3 other than the rear "shelf" area if it is a 78 or later, or the space behind the seats and under the rear deck on pre 78 models. either way, it isn't alot (of course much more on the 78-82 due to the "dome" created by the glass hatchback style). the only storage otherwise is a small box centered on the shelf as part of the storage tray area. the jack, etc fits in the passenger side "box" with a lift out tray about 3 inches deep and about 8"x10". the center compartment is a bit larger and is just storage. it is about 6" deep and the same approx 8"x10". the "box" behind the driver's seat houses the battery and has no additional space for use. the shelf on pre 78 C3 models is approx 8"to 10" high, runs the width of the car and is approx 2.5' or so deep. on a 78-82 the area is approx the same with the difference being the height which is limited only by the roof line at the halo section and my best guess would be that it is approx 3' high shelf to roof top. of course that slopes downward to approx 8" at the rear of the interior.
i hope that is the information you are looking for. it is all my best estimates. the exact dimensions can probably be found by examining the AIM for the particular year you are talking about.