William H. Wilson, 61, a lay leader from McAllen, Tex., was elected moderator of the Presbyterian Church (U.S.A.) at the denomination's 197th General Assembly in Indianapolis this week.

The annual deliberative body also formally installed the Rev. James E. Andrews, 57, of Atlanta as stated clerk, as the denomination's top administrative officer is termed.

Andrews was elected a year ago at the church's first assembly since Northern and Southern branches of the church, divided since the Civil War, reunited to form a single denomination of 3.1 million members. He has served 10 years as stated clerk of the Southern church.

As the 10-day assembly got under way this week, delegates began processing the "overtures" -- action proposals from units of the church around the country that will come before the entire body next week for debate and action.

Abortion drew the largest number of overtures, with petitions challenging and supporting the church's traditional prochoice position.

Another major issue before the assembly is the Life and Mission Statement, drawn up last year by a 94-member committee after study of questionnaires circulated throughout the denomination's 12,000 congregations. The statement seeks to define the role and mission of the reunited church, both in light of its history and in the context of the issues confronting the world today.

A crucial question before the assembly is the site for headquarters of the reunited church, which for two years has continued to operate out of the offices of the two parent bodies, in Atlanta and New York.

Moderator Wilson, a rancher and consulting geologist in the petroleum business, won majority support of the delegates on the first ballot. He was the only layman in the field of four candidates.

In the unpaid post as moderator, he serves for a year as titular head and spokesman for the denomination and is expected to travel extensively for the church.

Church officials have asked local Presbyterian churches across the country to hold special prayer vigils on June 16 for the safe return of Americans who are being held hostage in Lebanon, including a Presbyterian missionary, the Rev. Benjamin Weir. The date was chosen to coincide with the end of Ramadan, the Islamic month of fasting.