The top two teams in the National Hockey League begin a best-of-seven series for the Stanley Cup tonight, and that is the way it should be. Unfortunately for those who applaud close competition, one of the teams is the Montreal Canadiens.

To capture the Cup, the challenging Boston Bruins must win four games. That is the total number of defeats the Canadiens have absorbed in their last eight playoff series, over three years. Of their last 11 games against Boston, the Canadians have won 10, with the 11th tied, while outscoring the Bruins, 50-18. That includes a four-game sweep of the 1977 final series.

Still, the Bruins were bearing no white flags yesterday morning as they boared their Delta flight to Montreal, where the opener will be played tonight at 8, in the only playoff contest scheduled for Washington television consumption (WDCA-TV-20).

"I have confidence we're going to win it," said Peter McNab, the five-point hero of Thursday's clinching 6-3 semifinal victory over Philadelphia. "The key is the first goal. We have to get it and make them press a bit. The favorite usually has the pressure on them.

It's fine feeling to win the semifinals, but guys who have been there tell me that winning the Stanley Cup is a lot better than winning the semifinals. If we do win, they'll probably have to pick me off the ceiling."

"We won't go into the Forum in awe of those guys," said veteran goalie Gerry Cheevers, whose agile moves on two sore knees repeatedly foiled the Flyers. "If we play our game, if we come out shooting, if we just keep together what we've got, who knows what will happen?"

Coach Don Cherry is seldom at a loss for words, but he paused at length when he was reminded that he had repeatedly referred to the Bruins and Flyers as the NHL's second-and fourth-best teams. Did he still consider Boston second

"The Montreal Canadiens lost one game in the playoffs, how many have lost (one)?" Cherry finally replied. "I think we could be called second best. I know that will disappoint a lot of people.

"We know what to expect. We'll give it our best. After we beat the Flyers last year, the team thought it was their Stanley Cup. I don't hear that this time. The talk now is 'four more wins.' They'll be ready."

Cherry said he would use more players during the Montreal series, expanding his four-man defensive corps to at least five and utilizing his extra forwards on swing lines.

The Bruins beat Philadelphia physically, by pounding stars like Bobby Clarke, who was virtually useless Thursday after suffering a bruised hip when he was boarded by Terry O'Reilly in the first minute. The two teams clutched and grabbed and hooked and tackled throughout the series, largely without penalty as the officials elected to "let them play."

Montreal is a bigger, faster team than Boston, less vulnerable to the laying on of hands. If the Bruins can't catch them, they can't hogtie them, and in case things do get a bit rough, Montreal Coach Scotty Bowmon has concocted a peacemaking line of Gilles Lupien (6-foot-6, 212 pounds), Pierre Bouchard (6-2, 208) and Rick Chartraw (6-2, 210).

In the spirit of good fellowship, Philadelphia's Bob Kelly offered some advice to his conquerors: "Steal the Canadiens' skates and then pray."