Following the UPCNA's 1956 merger with the Presbyterian Church in the United States of America (the same denomination from which the OPC had separated from in 1936) to form the United Presbyterian Church in the United States of America, Clark joined the Reformed Presbyterian Church, General Synod in 1957.
Menyusul bergabungnya UPCNA 1956 dengan Presbyterian Church in the United States of America (yaitu denominasi yang darinya OPC had memisahkan diri pada tahun 1936) untuk membentuk United Presbyterian Church in the United States of America, Clark bergabung dengan Reformed Presbyterian Church, General Synod pada tahun 1957.