Sean, Firing order for 3.8 is 1,6,5,4,3,2.
3.4/3.5 engine F.O. is 1,2,3,4,5,6.
see the attached links for more details.
Check the attached links,instruction and guides, Good luck
I hope this helped you out, if so let me know by pressing the helpful button.
http://www.grandprixforums.net/threads/83892-2000-Pontiac-Grand-Prix-GT-3-8-firing-order
http://ricksfreeautorepairadvice.com/3-4-v-6-vin-e-firing-order/
http://www.autozone.com/repairinfo/repairguide/repairGuideContent.jsp?pageId=0900c15280217b34