A 3.4L engine is unlikely to be a direct bolt-in replacement for a 2003 Impala. While both are Impalas, the 2003 model year used either a 3.4L or a 3.8L V6, and the mounting points, accessories (alternator, power steering pump, etc.), and potentially even the transmission may differ depending on the exact year and trim level. You'd likely need significant modification to make it work, if at all.
It's far more practical to use a replacement 3.4L or 3.8L engine intended for that specific year and model. Trying to swap in a different displacement engine, even within the same car line, will involve substantial mechanical expertise and custom fabrication.
