The Stanley Cup finals, prolonged by a surprising 3-2 Ranger win in Boston on Tuesday, return to Madison Square Garden tonight for the sixth game of an unexpectedly close series with even the Boston sportswriters conceding the possibility of a seventh game.

The Bruins, now leading 3-2 in games, fell victim to overconfidence and a tenacious Ranger effort in Tuesday's game which saw some atypical Boston scoring difficulties, notably on the part of Phil Esposito, and the offensive resurgence of Ranger retread Bobby Rousseau, who scored both the tying and winning goals.

Tonight's game, which could be the roughest of the series, may hinge on goaltending, and the success of both team's power play. Gilles Villemure, who was brilliant in the New York nets in the Ranger win Tuesday, is the hot goalie for New York, but Ed Giacomin may be the sentimental starting choice. Gary Cheevers will probably start for the Bruins.

The Bruins have had trouble maintaining leads against the Rangers. The Rangers, thriving on their underdog status, tied the first game after falling behind 5-1, but lost, and won the fifth game after trailing 2-1. New York's biggest problem has been winning the close ones--three times losing to Boston by one goal margins.