Palestinian guerrilla leaders are expressing concern that they may be exposed to greater attacks in southern Lebanon even if Israel and Syria reach a peaceful settlement of the Middle East missile crisis.

If Israel decides to destroy the Syrian surface-to-air missiles in the Bekaa Valley, the Palestinians fear that they, too, will be targeted.

If Syria removes the SA6s peacefully; Palestinian leaders expect Israel to go back to its policy of air strikes against Palestinian positions in southern Lebanon, which has been interrupted the last four weeks to allow U.S. mediation in the crisis.

Salah Khalaf, better known as Abu Iyad, the third-ranked leader in the mainstream Fatah branch of the Palestine Liberation Organization, said in a magazine interview this week that if the missiles stay put, Israel "would try then to restore its stature by mounting attacks" on targets in the south, the Mediterranean port of Sidon and Beirut and assassination attempts against PLO leaders.

The PLO view had implications for any compromise settlement that special U.S. presidential envoy Philip C. Habib is thought likely to have worked out in his shuttle diplomacy here, acording to the Palestinians and even their enemies among the right-wing Christian militias.

They both argue that the broad outline of any agreement involves trading withdrawal of the missiles against ending Israel flights over the Bekaa Valley, but in no way infringes on Israeli overflights or air raids in the rest of Lebanon.

Given recent Syrian rhetoric claiming to defend the Palestinian cause, Damascus, too, might be drawn back into a larger conflict if it felt compelled to make good its promised protection of the guerrillas.

Some leading Lebanese politicians privately toy with a way to avoid these dangerous loose ends: Israel would forsake its air strikes and the Palestinians would stop shelling Israeili settlements in northern Galilee.

Such an arrangement, it is argued, would make good Israeli Prime Minister Menachem Begin's recent promises to northern Sraeli citizens that they would soon no longer be subject to such guerrilla shelling.

The key Israeli target, according to military attaches and the Palestinians themselves, is the Beaufort Castle built by the Crusaders which then and now has been valued because of its line-of-sight observation of the northern Galilee.

The Palestinians expect the Israelis to mount a major operation against the castle, hoping to capture and man it with Israeli-backed militiamen loyal to the renegade Lebanese Army major, Saad Haddad.

Concerned about such an attack, the Palestinians prevailed three months ago on Libya to provide SA9 missiles, long-range artillery and other weapons, according to informed sources.

Although the Israeli prime minister only recently revealed the presence of the Libyan weapons and about 100 technicians manning the more sophisticated materiel, the sources said everything was in place weeks before the Syrian missiles were moved into the Bekaa Valley April 29.