DETROIT, OCT. 8 -- General Motors Corp. and the United Auto Workers today reached a new three-year labor agreement granting most of the same sweeping job security provisions the union won in a contract with Ford Motor Co.

Some elements of the Ford agreement, finalized this week in nearby Dearborn, have been "tailored to meet the realities of GM," according to a source familiar with the contract.

But most of the nuts and bolts of the Ford pact -- designed to ensure specific employment levels over the next three years -- are in the GM contract, sources said.

GM and UAW officials declined to comment on their tentative agreement, saying that details of the proposal will not be released until Monday, when the union's 300-member GM bargaining council meets in Chicago to review the settlement plan.

Some 72 percent of an estimated 104,000 UAW-represented workers at Ford approved the look-alike agreement. And the speculation among sources familiar with the GM proposal is that that plan, too, will win widespread union acceptance.

But acceptability on Wall Street is another matter.

Ford, which has reduced its U.S. production work force by nearly 45 percent since 1978, is in a much better position to offer job guarantees than GM, according to many analysts.

Ford's earnings are better, $3 billion for the first half of 1987 compared with $1.9 billion for GM in the same period. Ford's production costs per car are 10 percent lower than GM's.

Thus, according to a number of analysts, it would be economically imprudent for GM to accept a similar pact.

But both UAW and GM officials said that their contract will be mutually beneficial.

"This settlement ensures unprecedented job security and provides significant economic gains for our hourly employes," said Alfred S. Warren Jr., GM vice president for industrial relations. "At the same time, we have established a framework which provides opportunities for General Motors to increase its competitive capability."

Like the Ford contract, the GM pact bars layoffs in all cases, with the exception of sales declines of GM vehicles produced in the United States. Sales downturns caused by vehicles made abroad but sold by GM in this country cannot be used to initiate layoffs.

Like the Ford pact, the GM contract also restricts the use of overtime and the reliance on outside suppliers to extend layoffs. And, like the Ford contract, the GM pact reduces the role of attrition in eliminating jobs.

For every two GM jobs lost through death, resignation or retirement, the company can reduce only one of its guaranteed job positions -- the same as in the Ford pact.

An estimated 335,000 active UAW-represented workers at GM, 8,370 of whom are employed at GM facilities in Maryland, Virginia, West Virginia and Delaware, are affected by the tentative agreement. But not all of those jobs are necessarily guaranteed over the next three years.

For one thing, GM retained the right to lay off 37,000 workers by 1989 at 14 plants and at the key divisions of two others scheduled to be closed by that year. Those closings and layoffs are not covered by the agreement because they were announced before July 27, when negotiations began on the tentative agreement reached today, according to sources.

There also is the fate of another 58,000 workers in GM's troubled components divisions. Prior to the talks, the company had identified 58,000 jobs in its components operations involved in the manufacture of parts that could be bought more cheaply from outside suppliers.

Under the terms of the tentative agreement, many of those jobs will be protected. But many also will be subject to buyouts under the proposed settlement, sources said.

GM did get strong agreements to reduce job classifications, which should increase management flexibility in job assignments. And there are a host of other measures within the contract, as there are in the Ford agreement, to encourage labor-management cooperation on matters of productivity and quality, sources said.

But, essentially, said one GM source, "you have the Ford agreement at GM."

Still, said UAW President Owen Bieber and Donald Ephlin, vice president in charge of the union's GM department, the new agreement should help to improve GM's competitive posture.

"We've maintained all along that both the workers and General Motors have a great deal to gain from enhanced job security," Bieber said. Workers at GM have been victimized by cutbacks and mounting layoffs "resulting from factors beyond their control," Bieber said.

"The new agreement here should help to turn that around by providing a stronger basis for job security and a stronger sense of attachment to this corporation and its future," Bieber said.

Another UAW official said, with some agreement from GM executives, that GM's acceptance of the Ford pact should serve notice that the company has radically changed its attitude in dealing with the union. Instead of adversaries, as so often has been the case in the past, the company and the union see themselves as partners whose futures are inextricably linked to each other's actions, UAWa and company sources said.

"That, more than anything, is the real significance of this contract," one UAW source said.

The GM and Ford agreements replace three-year contracts that originally expired at 11:59 p.m. Sept. 14. Both agreements were reached without a strike. The GM agreement, in a surprise to many observers, was reached without even setting a strike deadline.

In economic areas the two contracts are identical, sources said. Both GM and Ford workers will get a 3 percent increase in base pay in the first year of the agreement and 3 percent lump-sum bonus payments in the second and third years.

The lump-sum payments cannot be rolled into base wages, which means they cannot be used to boost pension and related benefits, sources said. That concession was aimed at helping Ford pay for its $500 million guaranteed employment program.

The same formula will be used to help GM pay for its employment security program, which reportedly will cost nearly $2 billion.

Both sides now are selling the contract to their respective constituencies, with the union having to persuade the rank-and-file at GM that the new agreement is as good as the contract won at Ford