Complex sentence: In addition to his service as mission president, he was president of the Palo Alto Stake, and a regional representative of the Twelve Apostles before his call as a general authority--specifically, an Assistant to the Quorum of the Twelve Apostles--in April 1970.
Split into simpler sentences: In addition to his service as mission president, he served in the church as president of the Palo Alto Stake and a regional representative. He was called as a general authority, as an Assistant to the Quorum of the Twelve Apostles, in April 1970.