Arlington Republicans yesterday overwhelmingly endorsed an insurance executive for a seat on the GOP-controlled County Board and threw strong support behind Ronald Reagan's presidential bid.

Washington Blue Cross Vice President John Simone Pace easily won the party's approval for the single seat on the five-member County Board that will be decided in the November elections.

Pace, a former Marine Corps captain who is making his first bid for public office, is likely to be opposed by John Milliken, a former aide to Rep. Joseph L. Fisher, a Nothern Virginia Democrat and former County Board member. Milliken, a former Arlington Democratic Party chairman, is expected to gain his party's backing at a meeting Tuesday.

Reagan's Arlington supporters yesterday gave the former California governor a clear victory in the state's delegation selection process. Reagan won 58.4 percent of the delegates whom the Arlington GOP will send to next weekend's 10th Congressional District convention.

Reagan, who has the support of virtually all of Virginia's top Republican officeholders, outpolled George Bush yesterday by 519 to 316 votes at the county convention.

Top Republican Party officials have said they expect Reagan to win virtually all of Virginia's 51 delegates to the National Republican Convention. Three of those delegates are to be selected at next weekend's GOP district convention and the rest at other district and state party meetings.

Jade West, an Arlington real estate agent and Reagan supporter, easily outdistanced Wes Corber, a party activist, 431 to 223, to win the chairmanship of the county Republican committee.

Outgoing Republican Committee Chairman Hal Ekern hailed the party's "renaissance" and a resurgence of conservative power in a county that has long been considered a liberal stronghold in Virginia.

"These are difficult times," said another speaker, state Del. Martin Perper (R-Fairfax), one of three Republican candidates for Fisher's congressional seat. "And we all know who has brought these difficult times upon us."

Perper's speech was partly over-shadowed by the appearance of small, remote-controlled helium balloon that circumnavigated the auditorium, flying banners for Frank Wolf, one of Perper's opponents in a June 10 primary.