Just a thought but the 2003 impala l36 can use both the standard 100 amp as well as the police package 120 amp. That's what I planned on switching to when my alt dies in a couple months. In theory it should fit. I suggest you run by a parts store and size it up. It's not super high output but just enough to push a bit more electronics or run an under drive pulley with no loss (still a slight gain) of amperage. Your voltage should be the same across the board with the alts.