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Dear Mr. Curran,
Thank you for posting your question to History Hub. The Cartographic holdings at the National Archives facility in College Park, MD include a series of Ship Engineering Drawings, 1942-1978 which may include C3 cargo ship plans. These are records from the Navy’s Bureau of Ships, so they may or may not include pre-modification plans. There are also some ship plans in Record Group 357 (Maritime Administration). I would suggest contacting the reference staff at email@example.com for further information.
Looking at records outside of NARA, the Independence Seaport Museum in Philadelphia has a collection called Sun Shipbuilding & Dry Dock Company records which includes C3 general arrangement plans from 1940.
Best of luck with your research, and thank you for using History Hub!