Rank and file members of the United Auto Workers union early today appeared to be narrowly approving a tentative contract agreement with General Motors calling for an estimated $2.5 billion in wage and benefit concessions.

UAW officials said the final vote will be announced today in Detroit. "The final vote is going to be close," said UAW spokesman Don Stillman. "We obviously would prefer a heftier margin of approval. But the key thing is whether or not the contract will be ratified." Stillman said he was confident the contract "would go over."

The Detroit Free Press today quoted an unidentified UAW official as saying the pact would be approved by a margin of 8,000 or 9,000 votes--about 52 or 53 percent. "We'll survive, but barely," the official was quoted as saying when the balloting entered its final hours.

The tenative agreement covers about 470,000 UAW members. During early voting, which began March 28, the union rank and file appeared to be approving the agreement by about a 65 percent margin. But the approval margin was sharply reduced by rejection votes at major GM plants in Lordstown, Ohio, and Linden, N.J.

In balloting Tuesday and Wednesday, UAW members at GM's Linden assembly plant rejected the cost saving proposal on a 2,673-to-493 vote, an 88 percent margin. Lordstown workers ignored the nearly 4,000 layoffs affecting their numbers and maintained their dissident tradition by spurning the agreement on a vote of 4,577 to 1,296.

By comparison, UAW members at Ford Motor Co. last February voted 76 percent in favor of accepting an agreement that would save Ford an estimated $1 billion in labor costs by Sept. 14, 1984. Stillman said yesterday that Ford workers believed their company had a better case for requesting concessions, because Ford lost $1.06 billion in 1981.

GM, the nation's leading automaker, made a relatively modest $333 million profit last year.

Stillman said he was not surprised by the heavy vote against the GM proposal. "I knew that many of our GM members believed the company didn't need concessions," he said.

UAW and GM officials, nonetheless, hoped that the layoffs and other ills affecting the domestic auto industry would have produced a more enthusiastic response for the kind of labor-management cooperation--wage and benefits concessions for job security--that the tentative pact is supposed to represent.