A major iceberg lettuce company and its workers have extended their labor contract for six years. About 3,500 full-time and seasonal employees will get hourly raises of up to 80 cents an hour over the next six years in the extended contract with Bud of California, a subsidiary of Salinas-based Dole Fresh Vegetables. The new agreement also improves job security, said Michael Johnston of Salinas-based Teamsters Local 890. Johnston said the company asked the Teamsters to consider extending the previous six-year contract, which expires in September 1998, rather than negotiate a new one. Johnston said contract extensions that long are rare, but the union decided that the new deal was in the best interest of its members. Bud's lettuce harvesters now receive $7.40 an hour.