Pentagon leaders told the Air Force last week to assume that President Reagan would scrap the MX land missile, build a fleet of B1 bombers and put a fence of AWACS warning aircraft around the continental United States.

The assumption about the MX cancellation was for budget-planning purposes. Reagan has yet to make his decision on the future of the controversial missile. But the Pentagon planning underscores how far Defense Secretary Caspar W. Weinberger has gone to find an alternative to the missile.

Deputy Defense Secretary Frank C. Carlucci in one set of secret instructions told Air Force planners to assume that the MX would be replaced by a version of the Navy Trident 2 submarine missile, saving a total of $33.2 billion in the five-year period of fiscal 1983 through 1987.

At the same time, Carlucci continued in his guidance, research should go forward on other ways to base intercontinental ballistic missiles like the MX, including burying them deeper in the earth to withstand nuclear blast and stationing them aloft in giant aircraft.

Some Air Force leaders last night dismissed Carlucci's instructions of last week as a paper exercise, predicting that Reagan would end up building 100 MX missiles to be rotated among 1,000 cement garages on government land in Nevada. But they acknowledged at the same time that neither Weinberger nor Reagan has informed them that this "100-l,000" option has been adopted at the White House.

Other government officials said Weinberger is still fighting for the "common missile" option of an Air Force-Navy version of the Trident 2 already under development. Weinberger himself told the House Budget Committee yesterday that the MX decision has "virtually" been made and will be announced by the end of the month.

Weinberger long has warned that trying to deploy the MX as the Air Force recommended--2,000 missiles spread among 4,600 shelters in the valleys of Nevada and Utah--would bring an avalanche of lawsuits. The legal debris would delay the missile's deployment, he warned, at a time when Soviet warheads have become so accurate that they could destroy today's arsenal of U.S. land missiles, which are stationary underground.

"It's been Weinberger against the world," said one official in characterizing the defense secretary's stand against the Air Force's MX deployment plan.

Illustrating Weinberger's point, Gov. Robert List of Nevada in a letter to Reagan released yesterday said he wished to "reiterate my emphatic opposition to any MX deployment in Nevada." Earlier, the governor had indicated his state would allow the missile deployed there if the government said it was absolutely vital to national security.

Although former president Carter's Pentagon research chief, William Perry, recommended playing a shell game with a new land missile, he, like Weinberger, favored building the common missile rather than the MX. Perry said he lost his fight because the common missile "didn't look big enough. The generals said it looked like a peanut alongside Soviet ICBMs."

Under the Carlucci planning guidance sent to the Air Force last week, but subject to change, the Air Force would cancel the MX in fiscal 1983 and start building the Navy common missile that year with a down payment of $700 million. This amount would increase to $1.2 billion in fiscal 1984.

As a further hedge against U.S. land missiles being destroyed, the Pentagon plans to intensify its efforts to stop a bullet with a bullet, earmarking $1 billion for the Air Force's antiballistic-missile research in fiscal 1983 and 1984.

Carlucci's instructions also told the Air Force to assume for budget-planning purposes that Reagan would start building a fleet of B1 bombers in 1983 with $300 million, aiming toward having the first ones ready to fly by 1986. Air Force leaders have been predicting for months that Reagan will order 100 updated versions of the B1 bomber that Carter canceled in 1977 and generously fund development of the radar-eluding bomber called Stealth at the same time.

Clearer than Reagan's future course on the MX and B1 is where he wants to go on air defense. Documents circulating around the budget offices of the Pentagon confirm that there will be a reversal here. The Reagan administration wants to erect a tighter warning net against incoming bombers and missiles, even though experts have said in the past that this is a losing proposition.

The Pentagon's new strategic blueprint calls for building 12 Airborne Warning and Control Systems (AWACS) sentry aircraft to patrol the borders of North America. This is the same kind of plane the Saudis want to buy to guard against surprise attack. An AWACS orbits at an altitude of about six miles where its radars can detect aircraft trying to hide in the electronic clutter close to the ground that often foils ground radars.

In the belief the Soviets are progressing fast on a look-down AWACS of their own, Pentagon leaders shaping the president's military program for the next five years want existing B52 bombers armed with devices for countering Russia's airborne sentries.