The Reagan administration has embarked on a far-reaching program to rebuild Lebanon's armed forces from the ground up in the belief that the crisis there has presented it with "a window of opportunity" to bind Lebanon close to the United States and the West.

The Pentagon has decided to send American armored personnel carriers and artillery to Lebanese forces soon and to station American military officers in Beirut to coordinate the rearmament effort.

The administration also is considering an expansion of the 3,800-man multinational force now in Beirut, which includes 1,200 U.S. Marines, to make the force available for operations elsewhere in Lebanon by adding troops from several other countries.

One government planner said Lebanese President Amin Gemayel stunned State Department officials during his recent visit by stating in some private conversations that it might require as many as 60,000 troops from the United States and other nations to help stabilize his country.

Gemayel, who said publicly in Paris that he might need a multinational force of 30,000, evidently believes that much muscle is needed to ensure that Israeli, Syrian and Palestine Liberation Organization forces are withdrawn and to keep order after they leave, according to those familiar with the private talks in Washington.

They said he was especially anxious for the multinational force to help control the Beirut-to-Damascus highway, so neither Syria nor Israel would have an excuse to stay or clash again in the eastern part of Lebanon.

But top officials at the State and Defense departments, including the Joint Chiefs of Staff, remain opposed to increasing greatly the present U.S. participation in the multinational force, although Marine Corps planners are assessing in paper exercises what it would require to enlarge its contingent to 5,000 men or more.

Administration policy makers are aware that their hopes of strengthening Gemayel's internal position and maintaining U.S. influence with him will require the presence of some kind of multinational force, including U.S. troops, for some time to come, possibly two years or more.

In southern Lebanon, Israel's insistence on creation of a 35-mile security zone north of its border could lead, in the absence of such a force, to the Israelis maintaining de facto control over the region through the Israeli-backed Christian milita commanded by Saad Haddad, a cashiered Lebanese army major whose independence poses a threat to Gemayel's authority.

However, U.S. officials are still hopeful that, despite Israeli resistance, some or all of these functions, particularly in southern Lebanon, can be turned over to the United Nations International Force in Lebanon (UNIFIL). That would avert pressures to expand the separate multinational force in a substantial way and also limit how the force is redeployed outside of Beirut.

U.S. officials also insist that it still is too early to say what such redeployment might involve or how many more troops will be required to carry it out. That, they say, will depend on what kind of withdrawal plan emerges from the diplomatic negotiations only now getting under way, what arrangements are made for the security zone in southern Lebanon and, once these matters are settled, what peace-keeping problems remain that are beyond the capacity of the Lebanese army to handle.

Politically, administration leaders fear that Congress, which so far has been tolerant if not enthusiastic about the commitment of Marines to Lebanon, might balk at putting significantly more U.S. troops there. It may take some doing just to get Congress to go along with keeping the present force beyond the 90 days allowed by the War Powers Resolution; the administration may well be forced to ask for such an extension.

Putting a force in Lebanon by invitation is much easier than extracting them when conditions suddenly turn ugly and dangerous, Pentagon planners stressed.

Defense Secretary Caspar W. Weinberger also sees high risks of keeping Marines in Lebanon for an extended period and is pressing in policy discussions to let other nations provide any needed reinforcement of the multinational force in Lebanon, these planners said.

According to government officials, possible candidates for joining the U.S., French and Italian forces now in Lebanon include forces from Australia, Belgium, Britain, Canada, Greece and the Netherlands.

Morocco and Sudan offered to send troops to Lebanon early in the crisis, officials said, but Israel is certain to veto forces from Arab countries and the other nations are considered to be the more likely contributors now.

The Marines nevetheless are drawing up contingency plans in case they are directed to increase their force in Lebanon. Pentagon officials said they could go up to about 5,000 men, perhaps even 8,000, before straining their commitments inordinately.

But it was stessed both at the Pentagon and the State Department that these are currently paper studies and there is no crash effort to prepare to put a big force on the ground in Lebanon.

"The Joint Chiefs aren't working the problem," said one official in stressing that the administration hopes it will not have to provide much more of a presence than the one already in Lebanon.

In the longer range, the administration plans to send a Military Advisory Group to Beirut, probably including specialists from the Army, Navy, Air Force and Marine Corps, to work with Lebanese counterparts in rebuilding Lebanon's military by providing training and hardware.

Early shipments of American weapons to Lebanon are to include about 25 armored personnel carriers, with double that number to be sent later, 25 155-mm howitzers, and communications gear. M60 tanks also are expected to be sent later, but officials said the U.S. Army now has few to spare and the Lebanese have no immediate need for them.

In addition to the Military Advisory Group, the administration has decided to establish a U.S.-Lebanese planning group to assess continually the needs of the Lebanese military and plan the procurement of weapons far into the future.