Welcome to Stovebolt Doug!
Best suggestion I can start with is to suggest you post some pictures of "all" the ID plates, Chevy original and military as we can better see what you see and might offer some helpful info.
On your question about information sites to decode the DOD tags, unfortunately not really down to the nitty-gritty serial numbers. Once the vehicles are no longer part of the government inventory (usually sold of at auction), the majority of their historical records/files get disposed of/destroyed. Some folks like myself, who have had many past years experience with DOD equipment and vehicle data plates, can possibly offer some info once we can actually see the plates.
Most of the info was used to ID the equipment, by original DOD Contract purchase order numbers, military assigned make/model,classifications and sometimes special equipment if installed. There were also sometimes added plates for equipment issued to/assigned to specific installations for inventory and life cycle management of the equipment. Again, we can help best if we can see what you have.
Looking forward to seeing pictures of your project!
