Can't help you find the documents, but here are the document numbers.
Request for Proposal, DAAJOI-72-R-02S4(P401), "Utility Tactical Transport Aircraft System," 30 December 1971, revised 10 March 1972.
Prime Item Development Specification for Utility Tactical Transport Aircraft System, Specification No. AMC-CP-2222-31000, I March 1976.