In the 1960s, Stack helped build the collections of Josiah K. Lilly, which was donated to the
National Numismatic Collection at the
Smithsonian Museum of American History following Lilly's death in 1966, and
Louis E. Eliasberg, who accumulated the only complete collection of American coins prior to his death in 1976. He testified before the
United States Congress in 1973 in favor of the
Hobby Protection Act of 1973, which required replicas of coins to be "marked with certain identifying information in an effort to flag them as imitations'. He played a key role in pushing the American Numismatic Association to adopt the Sheldon Grading Scale in the 1970s. Stack served on the board of the
Professional Numismatists Guild for almost a decade, including as president from 1989 to 1991, and worked with the
American Numismatic Association to develop the standardized grading system in use today. In 1996, he again testified before Congress, this time the House Banking Committee, to propose the
50 State Quarters Program. ==Personal life==