There are copies of a May 1956 copy of the American "Trains" magazine's Big Boy special edition, along with a hardback copy of William Kratville's "Big Boy" book, which includes much detailed information, along with many photographs taken of the original locomotives being built.
Whilst there is a 3 1/2 inch gauge boiler design dated 1956, we suspect that the project started with copies of the works general arrangement, along with some other drawings of the full-size locomotive, which are included.