Yankee Stadium will have shorter fences in left and left centerfield next season, the idea being to help the home run production of New York's heavy concentration of righthanded hitters. The chief beneficiary should be outfielder Dave Winfield, who still has eight years left on his Yankee contract. . . . break up the cliques on the Montreal Expos, insiders say, and the Expos would be leading the National League East instead of struggling to look like contenders. Even though that charge is true, Montreal still has to get itself together defensively.
Baltimore manager Earl Weaver, who never misses a trick when it comes to stretching the percentages, has outdone himself this year by rotating John Lowenstein, Ben Ayala, and Gary Roenicke in left field. Their combined offensive figures make them one of the most productive ''left fielders'' in the American League.