Somwhere amongst the 17 pages of threads here in the "solid body and bass chat" section, there are at least one thread that goes into detail about using cad software for guitar bodies. There might be two threads, but since I dont use or have cad abilities I didnt pay much attention.
As far as plans and blueprints, If you make financial contribution to the webmaster, for keeping this site alive, you can get access to secret confidential items that non-contributors dont get. Among these items are drawings / blueprints.
I hav'nt contributed yet, as x-mas has left me broke, so I dont know for sure if the SG is in their. (there is a thread about this too) But if worse comes to worst, find a music store with a return policy, buy an SG, trace it, make your measuremnts, then return it.
Sorry I cant be of any real use to you on this subject