Dear Mr. Schneider,
Thank you for posting your request on History Hub!
We searched the National Archives Catalog and located a series titled Organizational History Files of Various Army Units, 1920-1972 in the Records of U.S. Army Operational, Tactical, and Support Organizations (World War II and Thereafter) (Record Group 338) but were unable to locate unit histories for the 351st Infantry Regiment from 1963-1965. Additionally, the series titled Unit Histories, 1943-1967 already has been searched for records of the 351st Infantry Regiment. Please see the response here.
It is possible that further extended searching by you or your representative would eventually yield some pertinent documents. RDT2 will be happy to make appropriate records and/or finding aids available in the Textual Research Room should you have an opportunity to visit the National Archives at College Park.
We hope this is helpful!