You know, the folks at the Boeing Aircraft Company may have some B-377 plans or materials. I found them pretty responsive in the past to inquiries from the public.
There are also some real live derivatives of the B-377 in existence. One might write to those air museums to see if they have any measured drawings. Three are: The Miniguppy in Tillamook, Oregon,
Aero-Spacelines Mini-Guppy — Tillamook Air Museum, converted from a PanAm B377, and KC97 aerial tankers in the Air Force Museum in Dayton Ohio, and March Field Museum, in Riverside California.