I've taken some measurements from my '61 Overlander. I'm not sure how useful they'll be to you though. I'm in the planing stage myself and I'm surprised just how much the design will be influenced by the wheel wells along with the tanks (black, grey and fresh).
My measurements are by tape and are rough but I intend to use them in Sketchup much like others have done to get a design model. Let me know if you need something specific.
Inside length 274", inside width 85" I didn't take the corner radius yet but IIRC it's close to 22". (excuse the Imperial I've been away from the UK a few years now).
1960 Sovereign 33' Pacific Railroad Custom