The world record for a fine arts auction was smashed with the aid of Japanese dealers whose bidding helped bring prices at a two-day sale of Impressionist and Modern art to an estimated total of $83.6 million. The auction (begun Monday) at Sotheby's in New York set a price record for nine artists, including Claude Monet, whose "Bridge in the Garden," was sold to a New York dealer for $2.87 million, breaking the Monet record of $2.64 million. But Japanese buyers dominated. "Japanese buyers accounted for one-third of the dollars spent," said David Nash, Sotheby's expert on Impressionist and Modern paintings. So far, 111 Impressionist paintings have been sold for a total of $63.6 million.