Looking at your list item by item.
Cold air intake: My preference would be FWI, it frees up more engine bay space and moves out of the way easier when you work on the TB area.
180* thermostat --> could i run a 160 thermostat?: There is no good reasoning I can think of that you'd benefit from a 160. I personally am still running a 180 undrilled high flow Intense-Racing stat.
3.4" pulley: Good pulley, you may want to start at 3.5 and once you can tune work your way down to a 3.4.
colder spark plugs: Autolite 104'* would be a good choice
PCM upgrade: please elaborate more. Custom tune or off the shelf?
Shift kit ---> necessary?: Nothing is technically necessary. You will want faster and firmer shifts though. Expect the cam is going to add a lot of power and your trans is going to take a beating until it dies.
Series 40 mufflers: IMO this has nothing to do with performance, only noise levels. Considering a GXP exhaust to enlarge the piping may gain you more, the car will sound different with an FWI and cam than it does now. I'd suggest holding off on the mufflers until the other mods are done. You may dislike the amount of drone that a stock exhaust can give off, and wantt o commit suicide over what 40'* do to it.
Headers: Vroom vroom, don't forget the W body dipstick tube and 3" high flow catalytic converter.
XP CAM w/gaskets: IMO one of the two good choices in cam. If you pull the engine to cam it, consider the potential that a higher stall torque converter can offer. When going to the similarly profiled S1X cam, it was suggested I step up to a 3000 stall. Have never regretted that one bit. I fell in love with the power almost instantly. While you are in this deep, add an oil pressure increase kit to your list. Keeping the pressure up is a wise move in these engines.
What I don't see on your list is a transmission upgrade. The 4th gear clutch hub is a known weakness as is the input shaft. These show their age and weakness when you add a good amount of power as you are planning.
00 Regal GS: All kinds of cool stuff.