General Motors Corp., seeking to burnish its environmental image, is set to announce today plans to begin production of fuel-efficient versions of its full-size pickup trucks and its city buses. GM Vice Chairman Harry Pearce is scheduled to announce that the No. 1 auto maker plans within the next few years to begin selling so-called "hybrid" variants of its full-size pickups, in which an electric motor assists the gasoline engine, improving the truck's fuel economy, a GM official familiar with the matter said.