For this city, frustrated by its team's lack of success in the NBA playoffs, Philadelphia's stunning victory over Boston in Sunday's Eastern Conference final was the most important in the franchise's history.

Not even when Wilt Chamberlain led the 76ers to a victory over San Francisco for the NBA title in 1967 was the triumph more significant in terms of financial success and community relations. Only a few weeks ago, Harold Katz, the club's new owner, had talked openly of shaking up the team, of trimming costs, of putting a new face on a familiar product that sold only 3,000 season tickets for 1981-82.

During Sunday's telecast, CBS announcers had even reported on the possibility of the team moving.

"It was a franchise-saver, no doubt about that," General Manager Pat Williams said of the 120-106 victory over Boston. "It was a rescuer. It turned the team around. The city is alive again. We sold 30,000 tickets Monday to the first two games (of the championship series against Los Angeles, starting here at 9 p.m. Thursday, WDVM-TV-9)."

Williams, a holdover from Fitz Dixon's regime, is working without a contract. Coach Billy Cunningham has one year left on his, and his job might have been in jeopardy if his team had blown a 3-1 lead in a best-of-seven series with the Celtics for the second year in a row.

Several players, most notably Darryl Dawkins, would be on the market today if the 76ers' season had ended Sunday.

"If we had lost, this entire franchise would have been very, very shaky," Katz said today. "We would have had to take drastic measures. You can't lose to Boston two years in a row after being up 3-1. We would have lost the confidence of the entire city. We probably wouldn't have sold a handful of season tickets.

"I live here, this is my city and basketball seems different than other pro sports," said Katz, 45, who made his fortune through a nationwide chain of weight-loss centers. "In basketball, it's a city of front-runners. Friday night, the fans were walking out of the arena, calling us chokers. Now we're heroes."

With the exception of Julius Erving, who today refused to critize the media for its harsh treatment last weekend, basketball heroes here are hard to come by.

The players who felt they had to tip-toe out of town Saturday after scoring only 11 points and losing a six-point lead in the fourth quarter of an 88-75 loss in the sixth game, were greeted by some 4,000 fans at Philadelphia International Airport on their return Sunday night.

"We won our way back into our city's hearts with this one," Williams said. "We've been searching for seven years to do that. We can, indeed, win the big one. There wasn't a soul in America outside of the guys on this team who thought we could do it.

"The monkey is off our back at last. We've been living with the ignominy of what happened last spring, living with it for a whole year, being reminded of it day after day after day."

Philadelphia's fans and its media have a reputation for being tough, cynical, unappreciative. Santa Claus was booed at an Eagles game and once it was written that a cure for cancer would be booed, too.

When the 76ers lost the sixth game after leading by 15 points, when they made only seven of 34 shots to set a playoff record for second-half futility, the fans and the press turned on them. It was so bad that Cunningham refused to speak to reporters Sunday after probably the biggest victory of his five-year coaching career, except for a brief obligatory remark.

"I was just as upset as Billy," Katz said. "The papers here were extremely negative. They had us buried already, talking about funerals in Boston. But the press here has been negative forever. It's the roughest. I don't think if Washington had a team with the 76ers' record the last six years, the press would be so negative.

"We have the best record in the league in the last six years (329-161); we've beaten Boston three of the four times we've played them in the playoffs. There is no reason to be so critical."

There were public apologies in the papers here Tuesday, with one columnist writing that eating his words wasn't very appetizing.

"We were outcasts in our own city," said reserve forward Mike Bantom, a native of Philadelphia. "We were called it all--outcasts, misfits, chokers, losers. We had been criticized unkindly, people were making fun of us."

Now, of course, the bandwagon is full again. The opening game of the championship series against Los Angeles tonight is sold out. The media is singing the praises of the 76ers' character, of their great determination in the face of overwhelming odds, of how they brought the Eastern Conference title back to town.

Some of the players are willing to forget.

"Sure, we've been labeled by the fans," Caldwell Jones said. "I hate labels, but I hold back my feeling because if we unleash our emotions in the direction of the public, then we're the bad guys, another label. That's the nature of the profession. You listen, then forget about it."

Others are not so forgiving.

"What we're saying now is to hell with you, to the people who thought we couldn't do it," Steve Mix said. "That includes fans at the Spectrum, fans across the country, writers, broadcasters, whoever. You know who you are--if it applies, you're included. If it doesn't, it's nice to have you aboard."