If you like EFI and feel comfortable with it a LS-x motor makes more power per cube than any other motor built by GM to date. The Impala engine bay will swallow a truck motor with no dificulty (something you can not do with a smaller car).
Interiors are not to tough. You just need a handful of cheap special tools to take everything apart since it was designed to pop together with push in plastic retainers. Ask specific questions about each part you want to remove as they all have different techniques and tools for removing the part so you can reuse them again.