Will a 96 Buick century 231 fit into 99 Chevy lumina?

No, a 1996 Buick Century's 3.1L (231 cubic inch) V6 engine will not fit directly into a 1999 Chevy Lumina without significant modifications, if at all. While both cars are GM products and might seem similar, there are differences in engine mounts, wiring harnesses, computer systems, and potentially other components that make direct swapping very difficult, if not impossible. The engine bay dimensions and ancillary component locations are different enough to create significant incompatibility.