The Century, to my knowledge only ever had the 3.1 motor in it. I never paid a lot of attention to them though, so maybe 3.8 was an option. You don't mention the year of the Century, so ... that makes it a little tough. Essentially though, I'd get a pcm programmed for the NA motor and whichever trans you choose. Keep the GTP wiring harness and pcm. It's almost everything you would need.

01 in 00. Drop it in and hook it up on the 00 harness/pcm