The Senate began debate on the Panama Canal treaties yesterday with predictions from the leadership that final votes won't come before late March or April.

Most of the senators who spoke yesterday agreed that the opening of the debate was a "historic" occasion, but there was no unusual tension or excitement in the chamber The first speakers generally reiterated points for or against the treaties that they had made before.

The best available count of senatorial intentions suggested that 62 or 63 senators have definitely made th decision to support the two treaties, provided they are slightly modified to clarify U.S. right., after the canal is turned over to Panama at the end of the century.

Sen. Majority Whip Alan Cranston (D-Calif.) was reliably said to believe that 70 or 71 senators will eventually support the treaties - which need the votes of two thirds of the Senate to be approved. If every member votes, 67 affirmative votes will be required.

Administration lobbyists feel Cranston's number may be too high, and they are anxious to avoid overoptimism.

Both the Senate leadership and the administration now are more concerned about possible amendments to the treaties than about outright rejection. "The real danger is a 'killer amendment,'" According to an aide to Howard H. Baker Jr. (R-Tenn.), the minority leader and a key treaty supporter.

What is feared by treaty proponents is an amendment that is irresistibly attractive to a majority of the Senate, even though it would require a new plebiscite in Panama, a reopening of negotiations or both.

The proponents will argue that the existing treaties - with small modifications they are prepared to accept - represent the best possible deal that can be made with Panama now, so nothing should be added to them.

"No treaty . . . could possibly remove every doubt or deal with every contingency," Sen. John Sparkman (D-Ala.) admitted yesterday in his introductory speech for the treaties. But said Sparkman, chairman of the Foreign Relations Committee and nominal floor manager of the treaties, these treaties "offer the best solution that could be negotiated."

Opponents disputed that assertion, and said the Senate should not be pushed into approval without exercising its best judgment as to how the treaties might be improved. Avoiding another Panamanian plebiscite or renegotiation is less important than "guaranteeing the security of the canal and our national interest," asserted Robert Dole (R-Kan.).

The treaty opponent most feared by both Baker and Sen. Robert C. Byrd (D-W.Va.), the majority leader, is James B. Allen (D-Ala.), one of the Senate's most conservative members. he is a master of Senate rules and procedures who loves to debate.

The one episode yesterday that suggested the potential gravity and excitement of the debate came soon after the treaties became the pending business at 12:30 p.m.

Allen raised his large frame from his aisle seat on the Democratic side to pose 17 parliamentary inquiries to Vice President Mondale, who had come to preside and deal with Allen's queries. Allen had prepared the inquiries in advance, and answers were written out for Mondale to read.

In an interview in his office at 7:30 yesterday morning, Allen said the purpose of these inquiries was essentially to get details of Senate procedure on record before the debate begins in earnest. But other senators were obviously discomfited, perhaps because they weren't sure what Allen was up to.

Just one of the queries settled a substantive point. Only the English language versions of the treaties would be before the Senate, Mondale ruled - not the Spanish. Allen and other treaty opponents contend the two aren't the same.

Allen told the Senate he thought "dozens" of amendments to the treaties ought to be considered. In the earlier interview he said he had prepared some amendments, but "largely I'm going to play it by ear." He said he didn't want to "telegraph my thoughts" in advance.

Allen said he was especially annoyed by the argument that the Senate shouldn't meddle with the treaties because this could upset Gen. Omar Torrijos, the Panamanian leader, perhaps forcing him into a difficult situation.

The rule seemed to be "clear it with Omar," Allen said, recalling the line from Franklin D. Roosevelt's administration - "clear it with Sidney" - that symbolized labor's influence on FDR as personified by Sidney HIllman.

Allen is expected to seek or support amendments that would deny use of the canal of U.S. enemies in an emergency, clarify any differences between the Spanish and English versions, in some way compensate the United States for loss of its original $319 million investment in Panama and permit the United States to base troops in Panama after 2000. He may also have many more proposed changes.

Allen, famous for one-man filibusters in the past, said yesterday a filibuster would serve no purpose in the treaties debate, and he promised not to engage in stalling tactics.

The question of the cost of the treaties to the United States was raised again yesterday by Sen. John C. Stennis (D-Miss.), chairman of the Armed Services Committee, which held its own hearings on the treaties.

Stennis held a press conference yesterday to release committee estimates that the treaties could cost $1.023 billion in lost revenue and new expenses. The Armed Services Committee hired an accounting firm to help it arrive at this figure.

Administration officials said the amount was much too high, and promised a high-level administration response sometime this week. The issue of cost has obviously troubled some senators in recent days. CAPTION: Picture, SEN. JOHN C. STENNIS . . . committee estimates $1 billion cost