Sunset Park was the heart of New York City’s industrial waterfront until the end of World War II, when building weapons, handling cargo and deploying troops had Bush Terminal and the Brooklyn Army Terminal employing tens of thousands of workers. But even as factories and wholesalers have left the south Brooklyn neighborhood, the manufacturing zoning created to protect them has remained. And there, at the southern edge of Sunset Park’s M-1 zone, one developer is planning a six-story building with a combination of commercial and community uses 731 61st Street.