Defense Secretary Caspar W. Weinberger vowed yesterday that the Reagan administration would step in if it felt Saudi Arabia were going the way of Iran.

"We would not stand by, in the event of Saudi requests, as we did before with Iran, and allow a government that had been totally unfriendly to the United States and to the Free World to take over," Weinberger said on "Face the Nation" (CBS, WDVM).

President Reagan would intervene "if there should be anything that resembled an internal revolution in Saudi Arabia, and we think that's very remote," he said.

He stressed that Saudi Arabia is a sovereign nation and should be just as free as Israel and other nations to buy American weapons, with no strings attached, when it is in the mutual interest of buyer and seller.

He said the Saudis had not rejected the idea of the United States' putting controls on the AWACS planes, such as keeping American technicians aboard. "They've rejected any limits on their sovereignty . . . . The practical fact is that there will be a strong American presence" to help run and maintain the AWACS in Saudi Arabia. Having AWACS on patrol over Saudi Arabia is in U.S. as well as Saudi interests, the secretary said.

Meanwhile, former president Richard Nixon, in a statement released by his New York office, said it was the "intense opposition from Israeli Prime Minister Menachem Begin and parts of the Jewish community" that was stalling the AWACS sale.

The former president's first foreign policy statement during the Reagan administration drew a sharp response from Jewish organizations. "Richard Nixon's singling American Jews out from the broad spectrum of opposition to the AWACS sale is at best mischievous, at worst, mean-spirited," said Nathan Perlmutter, national director of the Anti-Defamation League of B'nai B'rith.

As Weinberger talked about the importance of Saudi Arabia and the need to go through with the sale of AWACS warning planes to that country, another part of the Reagan administration national security blueprint took more hits--the proposal to base MX missiles in vacated Titan silos.

"It means," said Chairman John Tower (R-Tex.) of the Senate Armed Services Committee in objecting to the MX-for-Titan switch, that the Soviets "can knock you out with fewer missiles and have more missiles left over to address to other military targets or, perhaps, civilian targets.

"This recommendation came as something of a surprise to the Department of Defense . . . . The senior professional military people and technical experts were not consulted or not permitted to critique this plan or even asked to cost it out . . . . The administration should have opted for at least 100 missiles in 1,000 holes," Tower said on "Meet the Press" (NBC, WRC).

He found little fault with the rest of Reagan's strategic improvement program but warned that his committee would have to "look carefully" at how much it would cost to build 100 B1 bombers and develop the Stealth radar-evading bomber at the same time.

If it looks as if there is not going to be enough for both planes, the chairman said, "we would go to the Stealth because it would be newer and more advanced technology."

Weinberger conceded yesterday that putting 36 of the new 100 MX land missiles in rebuilt Titan holes could be called a "stop-gap" while the administration ponders how the remaining 64 missiles should be deployed. He said piling new concrete and steel around the MX would make the missiles "survivable for a number of years."

Specifically, Weinberger said, the Titan silos will be upgraded to increase the blast pressure they could withstand from 2,000 to 5,000 pounds per square inch. This compares with the 600 pounds per square inch for the garages the Air Force wanted to build for MX missiles spread around the valleys of Nevada and Utah.

Former defense secretary Harold Brown, appearing on "Issues and Answers" (ABC, WJLA), scoffed at that claim. "Even if the best possible job is done" to stiffen the Titan silos, he said, the Soviets by 1984 or 1985 will have nuclear warheads so accurate that they could blast a cavity all around the site. Missile, concrete and all would fall into it. "You can't harden a shelter enough so that it will survive being in the crater from a nuclear explosion."

Hostile fire came from another corner--that of former president Jimmy Carter's national security adviser, Zbigniew Brzezinski. He said putting MX missiles with their 10 accurate warheads into Titan holes would lead the Soviets to conclude that the United States war plan called for attacking their land missiles in a surprise first strike. This would be "destabilizing," he said in a CBS television interview.

As for going back to producing the B1 bomber, "It's a flying Edsel," Brzezinski asserted.